A U.S. tank company commander pleaded not guilty Monday at the opening of his military trial on charges related to the killing last year of an Iraqi man witnesses have said was critically wounded.

Capt. Rogelio Maynulet (search), 30, from Chicago, could face a maximum sentence of 20 years and three months if convicted of assault with intent to commit murder. Maynulet, wearing full dress uniform, stood as his lawyer, Capt. Will Helixon, entered his plea.

The judge in the case, Col. James Pohl, threw out a second charge of dereliction of duty after the defense argued that prosecutors had not specified which duty he neglected.

The charges stemmed from a May 21 incident when Maynulet was leading his 1st Armored Division tank company on a patrol near the city of Kufa, south of Baghdad, where heavy fighting had been reported.

They encountered a car thought to be carrying a driver for radical cleric Muqtada al-Sadr (search) and another militiaman loyal to the cleric. U.S. soldiers chased the vehicle and fired at it, wounding both the driver and passenger.

When a medic pulled the driver out of the car, it was clear he had suffered critical injuries, with part of his skull blown away, according to testimony heard during Maynulet's Article 32 hearing — the military's equivalent of a civilian grand jury investigation.

Maynulet's fellow officers said at the Article 32 hearing that he shot the man in an act of compassion to end his suffering.

Maynulet's command was suspended May 25, but he has remained with his unit, serving on the division's planning staff.

He has not commented on the facts in the case, but lawyer Helixon said when he was arraigned in December that "he maintains that his actions were justified."

It was not immediately clear whether footage of the incident taken by a U.S. drone surveillance aircraft would be declassified for public view at the court-martial, which is scheduled to run until Friday.

Also Monday, the court-martial began for a U.S. soldier accused of refusing to perform his duty as a mechanic. Spec. Blake Lemoine, 23, from Moraville, La., could face a year in prison if convicted. He is with the 5-96th Maintenance Company of the 16th Combat Support Group.

An Army statement said he was charged with refusing to perform his duty in Darmstadt, south of Frankfurt, "on diverse occasions between Jan. 10 and Feb. 15, 2005, despite being repeatedly ordered to do so by his commanding officer."

Lemoine said he had asked to leave the Army after returning from a yearlong tour in Iraq. He gave several reasons for his decision at a news conference last week that was sponsored by several anti-war groups.

In particular, he argued that his duties as an ordained pagan minister were in conflict with his job in the Army.