D.C. police are cautioning residents of several Northwest neighborhoods to be careful after a man was killed and two people were badly hurt in “sudden” and “unprovoked” attacks this week.

The violence took place in a small area east of Rock Creek Park, Chief of Police Cathy L. Lanier said Thursday.

A 66-year-old Denver man, Gary Dederichs, was found suffering from fatal head wounds Tuesday evening in an alley in the 800 block of Emerson Street. The next morning, police found a 53-year-old man with head injuries on Georgia Avenue. That night, a 37-year-old woman was found with serious head injuries in the 5600 block of Ninth Street.

“Walk in pairs or in groups if at all possible,” Lanier said at an afternoon news conference. “It appears, preliminarily, that all three victims were walking alone at the time of the attacks.”

Lanier said she wanted to make the community aware of the attacks and the potential for an emerging pattern of random violence.

Gary Dederichs, 66, a tourist visiting Washington from Denver, was slain April 24 in a the Petworth area of the District. (AP)

Police have not conclusively linked the attacks but said they think that all three victims suffered head trauma after being hit with an unknown object. There was no obvious reason for the attacks, and the victims did not appear to share anything in common apart from that they were walking alone.

Dederichs died of his injuries after he was taken to a hospital. Investigators have not determined a possible motive in the attack.

When officers found the critically injured man in the 4800 block of Georgia Avenue on Wednesday, police speculated that he had been hit by a car; only later did they realize that his injuries were the result of an assault.

After police found the third victim, they grew concerned that the cases might be connected. They said they had little information from the victims and no information about possible suspects.

“It’s been very difficult to get any information from the victims because of the nature of their injuries,” Lanier said.

Police officials were increasing patrols in the 4th Police District on Thursday evening, sending officers door to door to advise residents of the attacks and to search for witnesses, Lanier said.