The gunman who allegedly killed a security officer at Los Angeles airport carried a note saying he planned to murder "multiple" agents, prosecutors said Saturday as they announced charges against him.

A prosecutor told how Paul Ciancia allegedly opened fire at "point-blank range" on Gerardo Hernandez, the first Transportation Security Agency agent killed since the office was created after the Sept 11, 2001 terrorist attacks.

The dead officer's widow meanwhile said she was "truly devastated" by his death.

"He was a joyful person, always smiling and took pride in his duty to the American public," said Ana Hernandez.

Ciancia, who used a Smith and Wesson assault rifle and a large stock of ammunition in the attack, faces a possible death penalty on charges of murdering a federal officer and committing violence at an international airport.

The gun rampage Friday triggered chaos at LA international (LAX) airport, disrupting more than 1,500 flights. The airport only fully reopened Saturday, more than 24 hours later.

Describing how the shooting started, a prosecutor said Ciancia pulled a ".223-caliber M and P-15 assault rifle out of his bag and fired multiple rounds at point-blank range" at Hernandez, 39, shortly after 9:00 am in LAX's Terminal 3.

The shots wounded the unarmed TSA officer but did not kill him. The 23-year-old gunman, dressed in black and wearing a bullet-proof vest then went up a nearby escalator and came back down to "shoot the wounded officer again," said US Attorney Andre Birotte.

Ciancia then proceeded further into the terminal, shooting four more people including two other TSA officers, before being detained after a gunfight with airport police.

The alleged gunman, who remains in hospital and has not yet answered questions from investigators, carried a note in his bag indicating that "he targeted especially TSA officers," for whom he felt "anger and malice."

The handwritten note, found in a bag taken into the airport, showed that he "made a conscious decision to kill multiple TSA employees," said David Bowdich, the head of the FBI's Los Angeles office.

"In the note that was handwritten by the defendant, that was signed by the defendant, we found a statement where he made a conscious decision to kill multiple TSA employees," said Bowdich.

"He addressed them at one point in the letter, and stated that he wanted to 'instill fear into their traitorous (sic) minds.'"

The handwritten letter resembled a suicide note, an unnamed law enforcement official told the Los Angeles Times. The note also said that Ciancia did not want to hurt anyone "innocent," just TSA agents, the newspaper reported.

1,550 flights hit

Los Angeles Mayor Eric Garcetti said Friday that Ciancia had a lot of ammunition when he was arrested.

Birotte said Ciancia was shot four times by airport police, and that he had some 150 rounds of ammunition. Two LAX police fired at the gunman, according to the criminal complaint filed against him.

An estimated 1,550 scheduled flights with some 167,000 passengers were impacted Friday amid the resulting ground stop, they said.

During the shooting spree, which lasted less than 10 minutes, Ciancia approached a number of people cowering in the terminal and pointed his gun at them, asking if they "were TSA," according to the Los Angeles Times.

If they answered "no," he moved on, the Times reported, citing witnesses who said he cursed the TSA repeatedly.

"This act of violence reminds us of the risks the brave men and women of TSA face every day as they work to protect the traveling public," said Rand Beers, acting secretary of Homeland Security.

While officials have not released details about the suspect's condition, the Times said he was hospitalized in critical condition after being shot in the head and leg.

The FBI said Ciancia was a Los Angeles resident originally from the eastern state of New Jersey.

Before the shooting, Ciancia texted his younger brother that he might harm himself, The Washington Post reported. This led the shooter's father to contact local New Jersey police, who in turn contacted their counterparts in Los Angeles.

LAPD officers visited Ciancia's home on Friday but could not find him, according to the Post.

The shooting came just weeks ahead of the stressful end-of-year US travel period that includes Thanksgiving -- traditionally the busiest travel time of the year -- and Christmas.