"There are some specific components that we won't go into the details to protect the integrity of the investigation, but there are some specific things we have seen that make us believe all three of these packages were constructed by the same bombmaker," Manley said this morning.
Federal and local investigators are working around the clock, but at this point, there has been no break in the case, Manley told ABC News on Tuesday.
"The fact they have not only been able to build these bombs, but then travel with them to the target location without them exploding either during construction or during deployment, shows that they do have a certain level of sophistication and they know what they are doing," Manley said.
Austin police have urged residents to call police if they see any unexpected or suspicious packages near their homes that were left overnight and do not have labels from an official shipping source such as the U.S. Postal Service, UPS or DHL, prompting 265 such calls to authorities as of Tuesday afternoon, Manley said in a later press conference.
Investigators responded to each call but "have not found any additional package that was suspicious or involved in any way with the investigation that is going on right now," Manley said, vowing to not "leave any stone unturned" and follow up on every lead until the case is solved.
Each package was placed in a cardboard, but Manley instructed residents to not ignore a "suspicious package" just because it is in a "different type of container."
Austin Mayor Steve Adler reiterated the call for residents to remain aware and alert in their efforts.
"At this point what we want people to do is to be very vigilant," he said on "Good Morning America" Tuesday morning.
“We don't want anybody to open or pick up a package that in any way feels suspicious," he continued.
Here's what ABC News knows right now:
2 dead, 2 hospitalized
Two lives have been lost, and an "additional two" have been "forever changed" from three separate explosions, Manley said.
On Monday, 17-year-old Draylen Mason was killed in the blast, Manley said, describing Mason as "outstanding young man who was going places with his life."
"It is an absolute tragedy that he is no longer with us," Manley said.
Mason's mother, who is in her 40s, is in stable condition.
A 75-year-old Hispanic woman, whose identity was not released, is still in critical condition with life-threatening injuries after she was injured in a separate blast on Monday, Manley said.
Another victim, Anthony Stephan House, died March 2.
Each was harmed by one of the three explosive devices received as a "box-type delivery," Manley told reporters Monday.
In two of the explosions, the package detonated as soon as the victim picked it up, Manley said. In a third, the package was brought inside the house and exploded when it was opened.
The victims suffered "traumatic, penetrative injuries," Manley said.
The first blast was initially thought to be an isolated incident
When the incident occurred on March 2, investigators originally believed it to be an isolated incident without "any further concern or threat" to the community because of a bust that happened at a drug stash house three days prior, Manley said.
Because police took a "significant amount of cash" from the stash house, authorities initially thought the explosion "may have been a retaliatory act," but that the perpetrators got the wrong house, Manley said.
At the time, there was "no information to believe that it was related to a larger plan," Manley said.
Two others were reported Monday. "Significant damage" was inflicted by at least two of the blasts.
The victims noticed packages left on their front porches, without any suspect ringing the doorbell or otherwise alerting them, Manley said.
"We don't want to have people be overly alarmed," Manley said, "but it is important that people be vigilant and be aware of things that look suspicious."
Residents are being asked to report all suspicious packages. "If you see something that’s out of place, do not handle it, do not move it, do not touch it. Call us," he added.
The first explosion occurred around 6:44 a.m., police said. It was "very similar" to the March 2 incident, Manley said. The device was left in front of a single-family home in the northeast part of Austin.
The second attack Monday happened around 11:50 a.m.
"The victim in this incident came outside of her residence and found a package out front and picked up that package," Manley said.
Police are not ruling out terrorism as a motive, either, because they don't want to "limit" what they are investigating, Manley said.
Evidence is being collected and sent to the ATF lab in Walnut Creek, California, so investigators can attempt to reconstruct what was in the explosive device, an ATF special agent said Tuesday afternoon.
Texas Gov. Greg Abbott has offered a $15,000 reward for information leading to the arrest of the person or people responsible for the explosions.
“I want to assure all Texans, and especially those in Austin, that local, state and federal law enforcement officials are working diligently to find those responsible for these heinous crimes,” Abbott said in a statement.
The ATF, FBI and Austin Police Department are offering an additional up to $50,000 reward for information leading to the arrest and conviction of those responsible. The Texas Department of Public Safety and the U.S. Postal Service as also assisting on the task force team, as well as the Travis County district attorney, who is "setting this case up for prosecution" when a suspect is captured, Manley said.
ABC News' Alex Perez and Gina Sunseri contributed to this report.