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.
Ciancia is facing charges of murder of a federal officer and committing violence at an international airport. The charges could qualify him for the death penalty.
It was not immediately clear when he would make a first court appearance given his medical condition.
In court documents and interviews, authorities spelled out a chilling chain of events, saying Ciancia walked into the airport, pulled a .223-caliber assault rifle from his duffel bag and fired repeatedly at 39-year-old TSA officer Gerardo I. Hernandez. He turned back to see Hernandez move and returned to finish him off, according to surveillance video reviewed by investigators.
He then fired on two other uniformed TSA employees and an airline passenger, who all were wounded, as he moved methodically through the security checkpoint before airport police shot him as panicked travelers hid in stores and restaurants.
It wasn't clear why Ciancia targeted TSA officers, but what he left behind indicated he was willing to kill any of them that crossed his path, authorities revealed.
The shooter's duffel bag contained a handwritten letter signed by Ciancia stating he'd "made the conscious decision to try to kill" multiple TSA employees and that he wanted to "instill fear in their traitorous minds," FBI Agent in Charge David L. Bowdich said.
"Black, white, yellow, brown, I don't discriminate," the note read, according to a paraphrase by a law enforcement official briefed on the investigation. The official spoke on the condition of anonymity because he was not authorized to speak publicly.
The screed also mentioned "fiat currency" and "NWO," possible references to the New World Order, a conspiracy theory that foresees a totalitarian one-world government.
The letter also talked about "how easy it is to get a gun into the airport," the law enforcement official said.
When searched, the suspect had five 30-round magazines, and his bag contained hundreds more rounds in boxes.
U.S. Rep. Michael McCaul, R-Texas, chairman of the House Homeland Security Committee, said on CNN's State of the Union on Sunday that Ciancia's actions show how difficult it is to protect travelers at a massive airport such as LAX.
The terminals are open and easily accessible to thousands of people who arrive at large sliding glass doors via a broad ring road that fronts the facility and is designed to move people along quickly.
"It's like a shopping mall outside the perimeter, it's almost like an open shopping mall," McCaul said.
The FBI has served a search warrant on a Sun Valley residence where Ciancia lived, Ari Dekofsky, a spokeswoman for the FBI's Los Angeles field office, said Sunday. Agents are still interviewing people, she said.
Authorities believe the rifle used in the shooting was purchased in Los Angeles. Ciancia also had two additional handguns that he purchased in Los Angeles, but which weren't at the crime scene, a law enforcement official said. The official, who has been briefed on the investigation, was not authorized to speak publicly and requested anonymity.
The purchases themselves appeared legal, although authorities were still tracing them, and it's unclear if the shooter used his own identification or someone else's, the official said.
"He didn't buy them on the street. He didn't buy them on the Internet," the official said. "He bought them from a licensed gun dealer the rifle and the two handguns."
Hernandez, a three-year veteran of the TSA, moved to the U.S. from El Salvador at age 15, married his sweetheart, Ana, on Valentine's Day in 1998 and had two children.
The other two TSA officers wounded in the attack have been released from the hospital.
Brian Ludmer, a Calabasas High School teacher, remained in fair condition at Ronald Regan UCLA Medical Center with a gunshot wound to the leg. Two other people suffered injuries trying to evade the gunman, but weren't shot.
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.
Associated Press writers Alicia Chang and Gillian Flaccus in Los Angeles contributed to this report.