Salvador Ramos, the suspect in the Robb Elementary School shooting, after turning 18 on May 16, purchased two rifles and carried out the second-worst school shooting in U.S. history within the span of eight days, according to multiple law enforcement officials.

The suspect purchased two AR-15-style rifles on May 22, two days before the massacre and six days after his birthday, multiple law enforcement officials told ABC News. They were legal purchases.

Once the shooter made entry into a classroom he barricaded himself and opened fire, according to the sources.

Officers from the Uvalde Police Department and agents from Customs and Border Protection entered into the classroom and immediately took fire from the gunman before they shot and killed him.

Investigators are going through the ballistics to determine who fired the fatal shot.

Ukrainian President Volodymyr Zelenskyy offered his condolences to the families of the victims of the shooting on Twitter.

"Deeply saddened by the news of the murder of innocent children in Texas. Sincere condolences to the families of the victims, the people of the US and @POTUS over this tragedy," Zelenskyy said in a tweet.

Zelenskyy added: "The people of Ukraine share the pain of the relatives and friends of the victims and all Americans."

Actor Matthew McConaughey decried the shooting in his hometown of Uvalde, Texas.

"We have tragically proven that we are failing to be responsible for the rights our freedoms grant us," he wrote in a statement on Twitter.

"We cannot exhale once again, make excuses, and accept these tragic realities as the status quo."

The 24-year-old National Youth Poet Laureate took to Twitter following the tragedy to share a poem.

She also posted a series of tweets on gun violence.

“It takes a monster to kill children,” she wrote. “But to watch monsters kill children again and again and do nothing isn’t just insanity -- it’s inhumanity.”

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When federal agents from Customs and Border Protection and Homeland Security Investigations arrived at Robb Elementary School Tuesday, they immediately encountered a scene of confusion and chaos, according to multiple law enforcement officials briefed on the attack.

Some arrived with heavy equipment, others with whatever they could carry as they ran to the scene on foot.

Initially, these agents, multiple law enforcement sources told ABC News, tried to discern who was doing what, where was shooter who was engaging law enforcement and how were children were being evacuated.

Once the tactical team assembled with enough gear, they became aware of an order not to enter the classroom because they were told the suspect had barricaded himself, the sources told ABC News.

The Texas Department of Public Safety has said the incident commander wrongly believed the shooting had stopped.

Eventually, the special agent-in-charge of Homeland Security Investigations gave the instruction to all federal agents under the umbrella of the Department of Homeland Security that they were free to use their best judgment and to do what they felt was best, the sources said.

The federal agents were unsure whether any children could be saved at that point, but they were interested in evacuating the wounded. Some agents brought children in other classrooms out through windows.

The tactical team went in at 12:50 p.m. CDT and fatally shot the suspect, 77 minutes after the shooting started, officials said.

The suspect was dead at the scene, the sources said. He was found with more than a dozen bullet wounds.