The Army said yesterday it knew for more than a year after 1st Lt. Kenneth Michael Ballard's death in Iraq in May 2004 that he was not killed in action, as it initially reported. The family was not told the truth until Friday.

Ballard's mother, Karen Meredith, of Mountain View, Calif., said in a telephone interview that she is angry and will press for a full explanation. She is a public critic of the war and has attended antiwar protests in Crawford, Tex., outside President Bush's ranch, with grieving mother and peace activist Cindy Sheehan.

Meredith said she blames the Army's error on official incompetence, not an intent to cover up the truth.

"This news is stunning to me," she said. "People in the Army knew this news for 15 months, and why they couldn't be bothered to tell me the truth when this first happened and to have me go through this pain 15 months later is unconscionable on the part of the Army. It's a betrayal to my son's service," she said.

A letter from Army Secretary Francis J. Harvey was hand-delivered to her Friday. She said Harvey wrote, "I sincerely apologize to you for the unfortunate series of events that resulted in your not being informed."

Army officials said the failure to notify the family of the true cause of Ballard's death was an oversight. The military sometimes incorrectly categorizes the cause of war deaths. What is unusual about the Ballard case is that the error was recognized early but not reported to the family for more than a year.

On Memorial Day in 2004, the day after Ballard died, the Army informed his family that he had been killed by enemy fire while on a combat mission in the south-central Iraqi city of Najaf. In a casualty announcement from June 1, the Pentagon said Ballard died "during a firefight with insurgents."

The Army publicly disclosed on Saturday that Ballard, 26, actually died of wounds from the accidental discharge of a M240 machine gun on his tank after his platoon had returned from battling insurgents in Najaf. The tank accidentally backed into a tree, and a branch hit the mounted, unmanned machine gun, causing it to fire, said Col. Joseph Curtin, an Army spokesman. Ballard was struck at close range.