The slaying of 17-year-old Trayvon Martin, a Florida high school student who was shot and killed by George Zimmerman, a self-appointed neighborhood watch captain, has captured national attention.

Petitions and protests calling for justice for Martin have exploded amid allegations of racism and scrutiny into how local police  handled the investigation. George Zimmerman has yet to be charged in the case.

Below is a timeline of events. ABC News' Seni Tienabeso and Candice Smith contributed to this report:

Feb. 26: Trayvon Martin, a 17-year-old Florida high school student, is found shot and killed in Sanford, Fla., a community north of Orlando.

Several eyewitnesses report to police that they heard a scuffle, then a cry for help, and then a gunshot.

According to the Sanford police report, George Zimmerman, 28, a self-appointed neighborhood watch captain, is found armed with a handgun, standing over Martin. He has a bloody nose and a wound in the back of his head.

Martin is unresponsive and pronounced dead at the scene. He has no weapons on him, only a pack of Skittles and a bottle of iced tea, his cell phone and an ear piece.

Zimmerman tells police he killed Martin in self defense. Zimmerman's head and nose are cleaned up at the scene by EMTs. He is then cuffed and driven in a police cruiser to the Sanfrod Police department. He is questioned, tapes a video statement and is released. Police do not arrest him, nor administer a drug or alcohol test. But the police report classifies this as a homicide under statute 782.11 for the "Unnecessary killing to prevent unlawful act"

March 9: Trayvon Martin's family demands that police release the 911 tapes or make an arrest two weeks after Martin was killed. Police declined to comment at the time, but told ABC News the tapes would be released the following week.

March 13 : ABC News uncovers questionable police conduct in the investigation of the fatal shooting of Trayvon Martin.

Sanford Police Chief Billy Lee said there is no evidence to dispute George Zimmerman's assertion that he shot Martin out of self defense.

March 16: Martin's family first hears 911 calls from made the night George Zimmerman shot and killed Trayvon Martin. The cries for help send the boy's mother screaming from the room and prompted his father to declare, "He killed my son," a family representative tells ABC News.

ABC News affiliate WFTV publishes excerpts from the 911 calls.

One of several petitions for Zimmerman's arrest has garnered more than 250,000 signatures on a change.org site. At one point signatures were pouring in at the rate of 10,000 an hour, according to the website.

March 18: Trayvon Martin's family asks  U.S. Attorney General Eric Holder and the FBI to get involved in the investigation of their son's death.

March 19: A 16-year-old girl tells Benjamin Crump, the Martin family's attorney, about the last moments of Trayvon Martin's life. Martin was on the phone with her when George Zimmerman began following him. She recounted that she told Martin to run, then she heard some pushing, then the line went dead.

The U.S. Justice Department announces it has launched an investigation into Martin's slaying.

ABC News also learns that Zimmerman violated major principles of the Neighborhood Watch manual, which states, "it should be emphasized to members that they do not possess police powers, and they shall not carry weapons or pursue vehicles."

The State Attorney's office in Seminole County, Fla., announces that a grand jury will review the evidence of the case on April 10.

March 20: Sanford police department admits to ABC News that investigators missed a possible racist remark by the shooter as he spoke to police dispatchers moments before Trayvon Martin's killing.

March 21: Sanford city commissioners conduct a vote of "no confidence" against embattled Police Chief Billy Lee at a heated city council meeting. Three of five commissioners vote against the chief.

The city manager now decides whether or not to let Lee go.

Martin's parents join hundreds of protesters in New York City for the "Million Hoodie March," demanding justice for the slain 17-year-old.

The change.org petition calling for George Zimmerman's arrest has nearly 900,000 signatures and is now the fastest growing petition in internet history, according to change.org. Tweets from celebrities, such as Justin Bieber and Spike Lee, helped fuel wide interest in the case.

A public relations person for Benjamin Crump, the attorney representing the Martin family, tells ABC News they received 418 media calls in one day.

March 22: Sanford Police Chief Bill Lee announces he will "temporarily" stepping down amid accusations that his department bungled the investigation into the shooting death of Trayvon Martin.

Florida Gov. Rick Scott also announces State Attorney Norman Wolfinger, another key investigator tied to the case, has agreed to withdraw.

Martin's family meets officials from the U.S. Justice Department.

Thousands rallied in Sanford, Fla., organized by the Reverend Al Sharpton, to demand George Zimmerman's arrest. Sanford police continue to accept Zimmerman's claim that the shooting was in self defense.

March 23: Roughly 50 schools in Florida stage walkouts to protest the killing of Trayvon Martin and show support for the  change.org petition, demanding George Zimmerman's arrest.

The change.org online petition surpasses 1.5 million signatures, making it the all time biggest and fastest growing petition in change.org's history, according to the website.

At a White House press conference, President Obama takes time to address the Trayvon Martin case, saying, "If I had a son he'd look like Trayvon."

" Hoodies on the Hill," a group of Capitol Hill staffers, rally in support of Martin.

Fox News contributor Geraldo Rivera ignites a firestorm of criticism when he seemed to indicate that Martin's hoodie, which he was wearing the night of the shooting, was to blame for his death.

A second "Million Hoodie March"  takes place in Philadelphia. Thousands attend.

Members of the Miami Heat basketball team dispatch Twitter pictures, showing team members wearing hoodies in support of Martin.

March 24: A friend of George Zimmerman's family tells ABC News that the voice heard howling on the 911 tapes was Zimmerman's, not 17-year-old Trayvon Martin. The friend also said  Zimmerman "couldn't stop crying" in the days following the shooting.

Zimmerman's attorney Craig Sonner tells ABC News that Zimmerman is not a racist, and that his client has received numerous calls and  threats.

The  Black Panthers put up a $10,000 bounty for Zimmerman's "capture."

March 25: Joe Oliver, who describes himself as a close friend of George Zimmerman's family, appears on "Good Morning America" and reiterates that Zimmerman has gone into hiding, fears for his life and is "just now becoming aware of how big this has gotten."

Oliver later tells ABC News' David Muir that he "didn't even know whether George pulled the trigger."

Zimmerman's attorney Craig Sonner tells ABC News it will be clear in court that Zimmerman acted in self defense, and that he suffered a broken nose and an injury to the back of his head the night Trayvon Martin was killed.

March 26: George Zimmerman originally told police in a written statement that Trayvon Martin knocked him down with a punch to the nose, repeatedly slammed his head on the ground and tried to take his gun, a police source tells ABC News.

Lead prosecutor and veteran State Attorney Angela Corey tells ABC News that convicting Zimmerman won't be easy because of Florida's "stand-your-ground" law.

"The stand-your-ground law is one portion of justifiable use of deadly force," Corey said. "And what that means is that the state must go forward and be able to prove it's case beyond a reasonable doubt… So it makes the case in general more difficult than a normal criminal case."

ABC News confirms that Trayvon Martin was suspended from school for 10 days for possession of marijuana in mid-February. No comment from the family.

Reverend Al Sharpton announces he  vows to "occupy" Sanford, Fla., over Easter.

March 27: ABC News confirms that the night Trayvon Martin was shot, Chris Serino, the lead homicide detective on the case,  said he "disbelieved" George Zimmerman's testimony and recommended in an affidavit that Zimmerman be arrested for manslaughter. But the State Attorney's office, headed by Norman Wolfinger, instructed him not to press charges because it was deemed there wasn't enough evidence to lead to a conviction.

ABC News also confirms that Trayvon Martin was suspended from school three times. One of the suspensions involved graffiti. When a school guard looked through Martins' backpack he found items of women's jewelry and what was called a "burglary implement," according to the Miami Herald.  Martin denies he stole. No charges were pressed, police never followed up, the paper reported.

About 200 to 250 protesters gather in front of the U.S. Justice Department to ask the department to charge George Zimmerman with a federal hate crime.

Martin's parents, Sybrina Fulton and Tracy Martin, attend a forum on racial profiling and hate crimes on Capitol Hill. Tracy Martin says he believes his son was racially profiled.

March 28: ABC News EXCLUSIVE: A police surveillance video taken the night that  Trayvon Martin was shot dead shows no blood or bruises on  George Zimmerman. The initial police report noted that Zimmerman was bleeding from the back of the head and nose. His lawyer later insisted that Zimmerman's nose had been broken in his scuffle with 17-year-old Martin.

Rep. Bobby Rush, D-Ill., dons a hoodie in support of Trayvon Martin and is kicked off the House floor for improper dress code.

March 29: George Zimmerman's attorney Craig Sonner first tells ABC News that should charges be brought against his client, he would personally deliver him to the Sanford Police Department.

April 2:  Enhanced video footage of George Zimmerman being taken into custody the night he was in an altercation with Trayvon Martin shows what could be an injury to the back of his head.

Vidor Friedman, the president of the Florida College of Emergency Physicians, reviews the enhanced video and tells ABC News that "if somebody had been beating his head against concrete I'd think we'd see more obvious scrapes." He added that Zimmerman's nose looks "clearly defined" and "it doesn't look like his nose was broken or badly broken."

The initial police report noted that Zimmerman was bleeding from the back of the head and nose. His lawyer later claimed that Zimmerman suffered a broken nose but was treated at the scene.

April 6: New analysis of calls to police made by George Zimmerman show a man who never offered up race as a descriptor of suspicious people in the neighborhood watch area until being prompted by a dispatcher.

April 9: Florida special prosecutor Angela Corey announces her office will not bring the Trayvon Martin shooting before a grand jury. The grand jury is scheduled to be convened April 10, but will not hear evidence about the shooting.

April 10: George Zimmerman's legal team announces t hey would no longer represent Zimmerman because they have lost contact with him.

Six shots were fired into an empty police cruiser overnight at the Retreat at Twin Lakes in Sanford, Fla, where Trayvon Martin was shot and killed. No one was injured in the 4:30 a.m. shooting and Sanford Police remove the vehicle and begin an investigation.

Mayor Jeff Triplett tells ABC News that the town has become a "kindling box" over the Martin case.

April 11: Florida special prosecutor Angela Corey announces George Zimmerman will be charged with second degree murder in the shooting death of unarmed teen Trayvon Martin and that "Mr. Zimmerman has turned himself in."

April 12: Attorney Mark O'Mara takes on the role of becoming George Zimmerman's legal counsel. O'Mara suggests to ABC News that Zimmerman may apologize to Trayvon Martin's family.

Zimmerman makes his first court appearance since the shooting death of Martin. He does not enter a plea and his attorney does not request bond. His formal arraignment is scheduled for May 29.

April 13: Court documents show that investigators believe George Zimmerman "profiled" Trayvon Martin, "assuming Martin was a criminal."

ABC News also learns that Zimmerman is being kept in isolation and spent most of his first night in jail, weeping. His attorney Mark O'Mara said Zimmerman is open to directly reaching out to Martin's family.

April 18: Circuit Judge Jessica Recksiedler, the Florida judge tapped to preside over George Zimmerman's trial for the shooting death of Trayvon Martin, is disqualified and replaced by Judge Kenneth Lester Jr.

April 20: George Zimmerman's bail is set at $150,000. He stuns the court by taking the stand and apologizing to Treyvon Martin's parents for killing their teenage son.

EXCLUSIVE: ABC News obtains a photograph showing the bloodied back of Zimmerman's head, which was apparently taken three minutes after  he shot and killed Martin. The photo could give credence to Zimmerman's claim that Martin had bashed his head against the concrete as Zimmerman fought for his life.

Click here to view the image. Warning: graphic content.

April 23: George Zimmerman is released from the Seminole County Jail around midnight on $150,000 bond.

EXCLUSIVE: ABC News learns Sanford Police Chief Bill Lee will officially resign today. He had been "temporarily suspended" last month.