BAGHDAD, Iraq -- All seven crewmembers and passengers aboard a Marine Corps CH-46 Sea Knight helicopter were killed when it went down in Iraq's Anbar province Wednesday morning, a senior U.S. military officer based in Baghdad confirmed.

It's the fifth U.S. helicopter lost in Iraq in just over two weeks.

The servicemembers' names are being withheld pending notification of next of kin.

Army Maj. Gen. William B. Caldwell, a military spokesman, said an investigation was under way to determine why the helicopter went down about 20 miles from Baghdad while it was on a routine operation.

In Baghdad, an Iraqi air force officer said it was downed by an anti-aircraft missile. And an al-Qaida faction has claimed it down the helicopter.

Other Marine aircraft were in visual contact at the time it went down, a U.S. official said. He said he did not know whether a distress signal was communicated by radio.

CH-46 Used Since 1960s

The Navy and Marine Corps have used the Boeing CH-46 Sea Knight helicopter since the early 1960s, mostly as a transportation vehicle and also in search and rescue operations. Its tandem rotors allow pilots to hover in fixed positions for long periods, even in heavy and unpredictable crosswinds.

At 84 feet 4 inches long, 16 feet 8 inches tall and weighing 18,000 pounds, the helicopter is one of the largest in military use. It can carry up to 6,000 pounds of cargo, or 25 combat-loaded troops, and has the fuel endurance to stay airborne for approximately two hours. In emergencies it can land on water and stay afloat for up to two hours.

Because of its tandem rotor design, the “Sea Knight” is an extremely versatile aircraft. It is able to excel in various flight maneuvers, such as rearward and sideward flight, while other helicopters are extremely limited. This makes the helicopter ideal for its primary Navy mission of vertical replenishment.

Military IDs War Dead

A U.S. Marine and a soldier were killed in Iraq Tuesday, military officials said. The Defense Department also identified seven previous casualties.

The Marine died from wounds suffered while operating in Anbar province. The soldier died during an enemy attack on a security post southwest of Baghdad. Their names are being withheld until their families can be notified.

Army Sgt. Randy J. Matheny, 20, of McCook, Neb., died Feb. 4 in Baghdad of wounds suffered when an improvised explosive device detonated near his vehicle. He was assigned to the 1074th Transportation Company in Sidney, Neb.

Army Staff Sgt. Ronnie L. Sanders, 26, of Thibodaux, La., died Feb. 3 in Baghdad of wounds suffered when an IED detonated near his vehicle. He was assigned to the 407th Brigade Support Battalion, 2nd Brigade Combat Team, 82nd Airborne Division in Fort Bragg, N.C.

Army Chief Warrant Officer Keith Yoakum, 41, of Hemet, Calif., and Army Chief Warrant Officer Jason G. Defrenn, 34, of Barnwell, S.C., died Feb. 2 in Taji, Iraq, of wounds suffered when their Apache helicopter was forced to land during combat operations. Both soldiers were assigned to the 1st Battalion, 227th Aviation Cavalry Regiment, 1st Cavalry Division in Fort Hood, Texas. The incident is under investigation.

Army Capt. Kevin C. Landeck, 26, of Illinois, and Army Staff Sgt. Terrence D. Dunn, 38, of Houston, died Feb. 2 in Baghdad of wounds suffered when an IED detonated near their vehicle. Landeck was assigned to the 2nd Battalion, 15th Field Artillery Regiment, 2nd Brigade Combat Team, 10th Mountain Division in Fort Drum, N.Y. Dunn was assigned to the 210th Brigade Support Battalion, 2nd Brigade Combat Team, 10th Mountain Division, also at Fort Drum.

Marine Cpl. Richard O. Quill III, 22, of Roswell, Ga., died Feb. 1 from a non-hostile cause in Iraq's Anbar province. Quill was assigned to 2nd Battalion, 4th Marine Regiment, 1st Marine Division, 1st Marine Expeditionary Force in Camp Pendleton, Calif.

More American troops were killed in combat in Iraq over the past four months -- at least 334 through Jan. 31 -- than in any comparable stretch since the war began, according to an Associated Press analysis of casualty records.