A battalion of more than 500 soldiers that includes 86 Maine Army National Guard members has arrived in Iraq for a minimum yearlong deployment.

The unit, the Wisconsin-based 121st Field Artillery Battalion, already has suffered a fatality, less than a week after arriving near Baghdad.

Army Spc. Stephen W. Castner, 27, of Cedarburg, Wis., was killed Monday by an explosive detonated near his vehicle in Tallil.

The Maine platoon attached to the unit is known as Security Force II and is made up of soldiers from all over the state. Its members are assigned to conduct force protection operations, convoy security and route clearance, according to spokesman Capt. Shanon Cotta of the Maine Army Guard.

Most recently, Security Force II was conducting “left seat-right seat drives,” or learning from the departing unit how to conduct tactical operations, Cotta said by e-mail Wednesday.

The battalion was laid over in Bangor on its way to Iraq last week. After their plane experienced mechanical problems, the unit members flew into Bangor International Airport late on the night of July 17. The soldiers stayed at local hotels before taking off the next day.

The battalion had departed from Camp Shelby, Miss., where they were trained in a variety of weapon systems, small unit tactics and convoy procedures, according to Cotta.

Castner’s father has raised concerns about the training at Camp Shelby that his son and other Army National Guard members received before being sent into combat, the Milwaukee Journal Sentinel reported Tuesday.

U.S. Rep. Jim Sensenbrenner, a Republican from Wisconsin, has requested an investigation into the alleged training failures – including that an insufficient number of Humvees were supplied and that battalion members were not adequately trained for duty escorting military convoys, the newspaper reported.

The Associated Press contributed to this report.