A University of Maryland student who was working as a guard on an armored truck to pay senior year tuition and hoped to become a Methodist minister was shot and killed today while he waited in the truck during a pickup stop in a shopping mall.

Robert Keyser, 21, who relatives said had been a quarterback on the football team at Baltimore's Polytechnic High School, died of a gunshot wound to the back of his head, which occurred during an apparent holdup about 9:30 a.m. at the Columbia Mall, police said.

Two bags of money were missing from the Loomis Armored truck in which Keyser was found, police said, but there was no sign of how the attack occurred.

"We don't know if a struggle occurred or how the gunman was able to get into the truck," said Chuck Jackson, state police spokesman. Jackson said more than 50 police officers and FBI agents cordoned off the northern end of the parking lot and searched the area for three hours but were unable to form "suspect descriptions or vehicle descriptions."

"We were unable to turn up anything. At this point, we are at square one," he said.

Shop owners and visitors to the 200-store mall in this planned community off Little Patuxent Parkway said other portions of the parking lot and the shopping area were unaffected by the search.

Representatives of Loomis Armored Inc. and Equitable Trust Bank, the bank where the pickup was being made, would not comment on the incident. The other guard, identified by police as Alfred John Cook, could not be reached for comment.

Police said the shooting occurred within minutes after Cook, who bank workers said regularly picked up money from the bank, entered a southern service entrance of the mall. Cook returned to the parking lot to find the truck missing. He called police, but before officers arrived Cook found the truck on the other side of the mall, several hundred yards away, police said.

Cook found Keyser in the back of the truck, police said. Troopers who arrived on the scene made futile attempts to administer first aid and cardiopulmonary resuscitation, police said.

Keyser's uncle and godfather, Robert Keyser of Perry Hall, described his nephew today as a "young man who was just trying to earn money to get himself through college -- as he did all the other years."

The younger Keyser, whose dark good looks earned him the nickname Magnum from relatives, had worked odd jobs through two years of a business administration program at the College Park campus of the University of Maryland and his most recent year at the Baltimore campus, his uncle said. He played on Little League baseball teams throughout his youth and "probably knew every Little League team for 10 miles around," his uncle said.

The older son of a tour bus driver, the younger Keyser decided to take the armored truck job in June because it "was good money and he wanted to make whatever he could," his uncle said. The student "recognized there was some danger involved with this job, but he thought everything would be fine," his uncle said.

Keyser was a member of Parkside Methodist Church in Baltimore. In recent years, according to his uncle, he had decided that he would prepare himself for the ministry after graduation and he was "checking out studies" at West Virginia Wesleyan College

"Something like this -- we never understand, his uncle said.