UPPER MARLBORO, Md. (WUSA) -- Two and a half years after a night of celebration in College Park turned into a night of violence, prosecutors continued laying out their case against two Prince George's County police officers.

Clearly, the most compelling witness of the day was a former Maryland state trooper who testified as an expert witness that officers Reginald Baker and James Harrison used excessive force when they repeatedly hit John McKenna, a University of Maryland student, with batons while McKenna was defenseless, and unarmed, on the ground.

The expert witness said McKenna was not compliant, but also not aggressive, when first approached by officers, and that Officer Baker was justified in forcing McKenna into a wall with his shield.

However, according to the expert witness, once McKenna was on the ground, the officers could have, and should have, stopped hitting him.