Former high school classmates of alleged Los Angeles airport shooter Paul Ciancia describe him as a quiet student who kept to himself, often turning down invitations to hang out with other people.

Federal prosecutors have filed charges of murder and commission of violence against Ciancia, who allegedly shot and killed a Transportation Security Administration officer and wounded two others and an airline passenger on Friday at the Los Angeles international airport. He could get the death penalty if convicted.

Ciancia, 23, was hauled away moments later on a stretcher and later heavily sedated for medical reasons, but not before he told investigators he had acted alone when he opened fire in the terminal, a law enforcement official who has been briefed on the investigation told The Associated Press on Sunday.

Ciancia, also told police a friend had dropped him at LAX on Friday. Officials do not believe that the friend knew of the shooter's plans. Ciancia arrived at the airport in a black Hyundai and was not a ticketed passenger.

Ciancia was under 24-hour armed guard at the hospital Sunday after being shot four times, the official said. He was sedated for medical reasons, the official said, adding that one gunshot to the mouth blew a molar out of his jaw.

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Prior to coming to Los Angeles in 2012 hoping to work as a motorcycle mechanic, Ciancia graduated in 2008 from Salesianum School, an all-boys Roman Catholic school in Wilmington, N.J., the Associated Press reports.

One of his classmates there, Joseph Conti, said Ciancia kept to himself and rarely spoke in class, except to make occasional sarcastic remarks about a teacher, the Philadelphia Inquirer reports.

"But I've seen him smile and laugh before," Conti said.

"He kept to himself and ate lunch alone a lot," David Hamilton, another former classmate at Salesianum, told the Los Angeles Times. "I really don't remember any one person who was close to him .... In four years, I never heard a word out of his mouth."

Ciancia played trumpet in the school’s band but “never was interested” in invitations from fellow band members to hang out, added Conti.

Other classmates said Ciancia was the only member of the school’s class of 2008 that did not attend college.

It was assumed that after high school, Ciancia would work at his father’s auto-repair shop in his hometown, Conti told the Philadelphia Inquirer. Residents there said Ciancia was deeply affected over his mother dying of multiple sclerosis a year after his graduation.

In December 2011, Ciancia graduated from Motorcycle Mechanics Institute in Orlando, Fla., said Tina Miller, a spokeswoman for Universal Technical Institute, the Scottsdale, Ariz., company that runs the school. He then headed to Los Angeles a year later.

Ciancia’s family had told Pennsville, N.J. Police Chief Allen Cummings early Friday afternoon that he had mentioned taking his own life in a text message to his younger brother, the chief told The Associated Press.

Cummings notified the LAPD, which in turn, went to Ciancia's residence. He was not there at the time, but his two roommates told authorities that he seemed fine, Cummings said. LAPD told Cummings that they would check on him again when Ciancia returned from work.

Forty-five minutes later, Cummings got a call about the shooting.

According to federal prosecutors, Ciancia was so intent on murdering TSA officers Friday that an officer he had already shot was still moving, he turned around, walked back down an escalator, and shot him again.

Authorities say Ciancia walked into Terminal 3, pulled a Smith & Wesson .223-caliber assault rifle from his duffel bag and fired repeatedly at point-blank range at a TSA officer identified as Gerardo I. Hernandez, 39, who was checking IDs and boarding passes at the base of the escalator leading to the main screening area.

The killing was "believed to be a premeditated act of murder in the first-degree," U.S. Attorney Andre Birotte Jr. said in announcing the charges.

After killing Hernandez, Ciancia fired on at least two other uniformed TSA employees and an airline passenger, who were all wounded. Airport police eventually shot him as panicked passengers cowered in stores and restaurants.

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The announcement of his charges came after authorities say they have found a note handwritten by Ciancia in which he describes his intent to kill TSA employees and “pigs,” a law enforcement official told The Associated Press. The handwritten note found in Ciancia's duffel bag and signed by him stated that he had "made the conscious decision to try to kill" multiple TSA employees and that he wanted to stir fear in them, FBI agent in charge David L. Bowdich said.

"Black, white, yellow, brown, I don't discriminate," the note read, according to a law enforcement official who spoke to the Associated Press.

The suspect's screed reportedly also mentioned "fiat currency" and "NWO," possible references to the New World Order, a conspiracy theory that foresees a totalitarian one-world government.

A law enforcement official who was not authorized to discuss the investigation publicly and spoke on condition of anonymity, said the note referred to how the gunman believed his constitutional rights were being violated by TSA searches and that he was a “pissed-off patriot” upset at former Homeland Security Secretary Janet Napolitano.

The FBI was still looking into Ciancia's past, but investigators said they had not found evidence of previous crimes or any run-ins with the TSA. They said he had never applied for a job with the agency.

"We are really going to draw a picture of who this person was, his background, his history. That will help us explain why he chose to do what he did," Bowdich said. "At this point, I don't have the answer on that."

Leon Saryan, 65, had just passed through security when he heard gunshots. He fled with a TSA worker, who he said was later wounded slightly, and managed to hide in a store. As he was cowering in the corner, the shooter approached.

"He looked at me and asked, 'TSA?' I shook my head no, and he continued on down toward the gate. He had his gun at the ready and but for the grace of God I am here to tell about it," said Saryan, of Milwaukee.

Los Angeles Mayor Eric Garcetti said the gunman had an additional 100 rounds of ammunition he did not fire.

President Obama called TSA administrator John Pistole to express his condolences to the families and friends of the TSA officer who was killed and the two other officers who were wounded Friday.

Hernandez is the first TSA officer killed in the line of duty in the 12-year history of the agency, which was founded in the aftermath of 9/11.

Friends remembered Hernandez as a doting father and a good neighbor who went door-to-door warning neighbors to be careful after his home was burglarized.

In brief remarks outside the couple's house, his widow, Ana Hernandez, said Saturday that her husband came to the U.S. from El Salvador at age 15.

"He took pride in his duty for the American public and for the TSA mission," she said.

The shooting, which sent passengers in the airport scrambling for cover, disrupted flights nationwide as planes headed for Los Angeles were temporarily grounded. LAX estimated 1,550 scheduled flights were impacted by the shooting and approximately 167,050 passengers were affected, including those diverted to other airports, held on the ground in Los Angeles or held at their originating airport.

Fox News' Adam Housley, Shayla Bezdrob, Edmund DeMarche, Karl DeVries, Shayla Bezdrob and The Associated Press contributed to this report.