It wasn't clear why Ciancia targeted TSA officers, but what he left behind made it clear he intended to kill any of them who crossed his path, FBI Agent in Charge David L. Bowdich said.
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," Bowdich said.
The letter also talked about "how easy it is to get a gun into the airport," the law enforcement official said.
U.S. Rep. Michael McCaul, R-Texas, chairman of the House Homeland Security Committee, said on CNN's State of the Union on Sunday he had seen the note and said 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. "So it's very difficult to protect."
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.