DENVER -- Investigators are still trying to determine why a prosecutor was shot and killed outside his northwest Denver home, police said Thursday.

Sean May, 37, was gunned down at about 6:20 p.m. Wednesday by a man in his late teens or early 20s, police said. The coroner said he was shot in the head and abdomen. He was pronounced dead at a St. Anthony's Medical Center.

Detectives have not said if May was shot because he was a prosecutor or if it was a robbery attempt or random shooting.

Witnesses said the gunman was seen running from the home. A massive search was under way.

May was chief deputy district attorney in Adams County and had worked as a prosecutor for about seven years.

May's wife is six months pregnant with their first child. They bought the home in 2005, according to public records.

"Sean was a talented prosecutor, a loving husband and a steadfast friend," Adams County District Attorney Don Quick said in a statement. "We are all shocked and devastated by this terrible tragedy. Sean was a part of our office family and the loss is immeasurable. Our thoughts and prayers are with his wife and family."

U.S. Attorney Troy Eid called May "an extraordinary prosecutor, public servant and friend."

Gov. Bill Ritter called the slaying "a terrible, senseless act of violence" and offered his sympathy to his family.

The gunman was described as a white or Hispanic male in his early 20s. He is approximately 5 feet 11 inches tall, with a medium build. He has dark medium-length hair and a dark mustache and goatee. He was last seen wearing a black baseball cap backward, a white short-sleeve T-shirt, and khaki- colored, knee-length cargo shorts and tennis shoes.

Anyone with information about the case is asked to call 911 or Crime Stoppers at 720-913-7867.