A University of Maryland student killed during a drug-related robbery Tuesday became the 11th homicide victim in Prince George's County in as many days this year, putting added pressure on the police department to come up with a way to stop the violence.

A 12th killing occurred Tuesday night when an occupant of a house in the Lanham-New Carrollton area shot an intruder who was trying to break in, county police said. But they said that based on preliminary reports, the killing appeared to be justifiable, carried out in self-defense.

The 11 other slayings since New Year's Day appear to be isolated and mostly unconnected to one another, making the situation even more difficult for police. New County Executive Rushern L. Baker III (D) and the County Council also are scrambling to respond to the killings.

Officials continue to stress that those slayings are not random and that people are not in any added danger.

At least nine of the victims in the first 11 killings were specifically targeted by their killers, police said. Six of those crimes were drug-related. One involved a child-custody dispute, and one is believed to have stemmed from a personal argument. Another is somewhat of a mystery, although investigators have evidence that the victim knew his attacker.

"It's . . . important for folks to know that the lifestyle of these victims has greatly contributed to where they are in life," said Deputy Police Chief Kevin Davis.

Police believe that one of the remaining cases is a gang attack on an innocent victim and the other is probably linked to gangs or a personal dispute.

So far, police have made arrests in three of the slayings.

"To commanders and detectives, it's mystifying. . . . There's no common set of circumstances that we can go after," said Maj. Andrew Ellis, the department's public affairs commander. "If they were all drug-related, we could hit it with the drug angle. If they were all robbery-related, we could start combing our files for robbery suspects that had been recently released from jail. If it was a gang war, we could go after the gang leaders. And so the fact that it's disconnected makes it much more difficult to prevent."

The violence is even more confounding because homicides in Prince George's continued their five-year descent last year, hitting 98. Ellis said some detectives cannot remember a time when there has been such a consistent rate of killings.

"It's truly out of the ordinary," he said. "There's just no reasonable or logical explanation for the number of homicides that we've had since January 1st."

In the Tuesday night incident, which occurred about 8:40 p.m., police said it appeared initially that an occupant of a house in the 7400 block of Riverdale Road fired a shotgun as an intruder tried to force his way through the door.