BAGHDAD (CNN) -- Eight U.S. troops were killed in Iraq on Monday in a deadly chain of events that began when a U.S. helicopter crashed, apparently shot down by small-arms fire, according to a U.S. military official.

A military vehicle rushing to the helicopter crash site was hit by an exploding roadside bomb, and a second "quick-reaction force" vehicle also was hit, the official said.

The two pilots of the Kiowa helicopter were killed in the crash; six soldiers died in the bombings of the two vehicles, and three others were injured.

The eight Memorial Day deaths occurred in volatile Diyala province between Baquba and Muqdadiya, the U.S. military announced on Tuesday.

The statement said all of those killed were from Task Force Lightning, the force that patrols northern stretches of Iraq, including Diyala.

U.S. commanders have expressed concern about a rise in violence and the growing presence of al Qaeda in Iraq militants, who have fled to Diyala from other regions of the country.

Also Monday, two other U.S. soldiers were killed by a roadside bomb while conducting a "combat security patrol" in southern Baghdad, the U.S. military said.

The U.S. death toll for May has risen to 114, making it the deadliest month so far this year.

The highest monthly death tolls for U.S. troops occurred in 2004 -- 137 in November and 135 in April.

Since the start of the war, 3,458 U.S. service members have died. Seven civilian contractors of the Defense Department also have been killed in the war.