Nadia McCaffrey said she wanted to protest against a Pentagon policy banning media coverage of the US's war dead.
Nearly a dozen reporters, photographers and television crews watched as the coffin of Army Sergeant Patrick McCaffrey, 34, was transferred to a hearse outside an airport cargo terminal shortly before midnight on Sunday, officials said.
"I don't care what President Bush wants," McCaffrey, told the Los Angeles Times. Patrick "did not die for nothing... The way he lived needs to be talked about. Patrick was not a fighter, he was a peacemaker."
McCaffrey was killed on 22 June along with 1st Lieutenant Andre Tyson, both members of the 579th Engineer Battalion, when they were ambushed by fighters near Balad, Iraq.
Both were promoted posthumously on Friday, Sergeant Andrew Hughan of the California National Guard said.
The debate over whether Americans should see the coffins of McCaffrey and other troops flared last April after The Seattle Times published a front-page photograph of caskets in a cargo plane in Kuwait and a First Amendment activist posted on his website dozens of similar images from Dover in Delaware, home to the nation's largest military mortuary.
Sunday night's brief ceremony, however, did not violate the policy because it applies only to military facilities. The airport and the California National Guard worked on Sunday to arrange the event.
"I don't care what President Bush wants"
The Pentagon's rules "are specifically for the airlift command, when the caskets are on the military plane," said Lt Jonathan Shiroma, spokesman for the California National Guard. "This is a commercial jet, so it's a different jurisdiction, so to speak. We cannot stop the media from filming."
McCaffrey's casket arrived aboard a commercial flight from Atlanta and was placed in a hearse by fellow members of the 579th Engineer Battalion, Shiroma said. It was driven in a motorcade to a funeral home in his hometown of Tracy, where services were scheduled for Thursday.
McCaffrey was a manager with a collision repair company in the Silicon Valley when he enlisted in the National Guard in the days following the September 11 2001 attacks. But he had grown disillusioned with his mission in Iraq, his family said.
For her part, Nadia McCaffrey said she planned to continue speaking out against the war.
"This is enough," she said. "We have to react."