American forces in Iraq had four deadly accidents, three of which may have been caused by friendly fire, officials said Thursday.

Defense Department officials said they were looking into the possibility that one U.S. fighter jet was downed by an American Patriot missile and that a second jet fired on Army ground forces.

Statements on the two accidents followed word that an Army Black Hawk helicopter went down over central Iraq on Wednesday, killing six soldiers and injuring four and leaving one missing. Officials said Thursday it was unclear what brought down the helicopter, which was hovering above a firefight between American and Iraqi forces.

On Thursday, a U.S. Army soldier was killed by other American troops as he was investigating a destroyed Iraqi tank, U.S. Central Command reported. The soldier, mistaken for an Iraqi, was killed at about 8:30 a.m. EST, Central Command said.

"There are portions of this battle that are enormously complex, and human beings are human beings," Defense Secretary Donald H. Rumsfeld said. "And things are going to happen, and it's always been so and it will be so this time -- it's always sad and tragic and your heart breaks when people are killed or wounded by (it)."

All the cases were being investigated.

The Pentagon reported that the number of American service members killed in action since the war began March 20 has risen to 53, of which 41 were hostile deaths. The others were the result of accidents or other non-hostile circumstances. The number of troops missing is 16 and seven are prisoners of war.

The death toll does not include the six killed in the Black Hawk crash because the Pentagon has not formally announced the number or identities.

An operation was under way to find the pilot of a U.S. Navy F-18C Hornet downed over Iraq late Wednesday, Iraq time. It was flying a mission from the USS Kitty Hawk aircraft carrier, and missile firings were reported at the time near where it was lost.

One Army soldier was killed and several were injured or missing after a possible friendly fire accident in which an F-15E Strike Eagle fired on ground forces. It happened Wednesday night near Karbala, 50 miles south of Baghdad, officials said.

"We'll have to investigate each one of them, see if it was a breakdown in our techniques or our procedures or if there was a technical breakdown that we have to shore up," Air Force Gen. Richard Myers, chairman of the Joint Chiefs of Staff, said in a Pentagon news conference with Rumsfeld. "We'll just keep working at it."

In the 1991 Persian Gulf War, 36 of the 148 American dead were killed by their own comrades.

This time, friendly fire has caused five of 27 British deaths. Dozens of U.S. troops have been injured by their own forces, and the military is still investigating the combat deaths of nine Marines near Nasiriyah on March 23.