D.C. officers investigate tools on the front porch of a home where a teenager was arrested April 27 in connection with the recent Petworth neighborhood attacks.Police say the 19-year-old arrested the previous night had attacked a woman with a hammer. (Linda Davidson/THE WASHINGTON POST)

After a man was killed and two other people injured in separate attacks in the District’s Petworth neighborhood this week — each victim apparently chosen at random and struck on the head with a weapon — police saturated the area Thursday night with officers in plainclothes and unmarked cars. They feared a serial assailant was at large.

The tactic bore quick results.

Some of the officers, patrolling in the 800 block of Gallatin Street NW about 9 p.m., heard a woman scream for help. Like the earlier victims, she had been grievously injured by a sudden blow to the head. As the officers rushed to her aid, police said, they noticed a man with a backpack staring at them from an alley.

When the officers stared back, police said, the man turned and ran. Minutes later and a few blocks away, after a frantic foot chase ended with the suspect in handcuffs, officers opened his dark-colored backpack and peered inside.

Just hours into what might have become a protracted hunt for a dangerous and unpredictable attacker, police said, they caught a key break: They found a claw hammer.

Michael Davis, 19, who lives with his grandmother in Petworth, was charged with aggravated assault in Thursday night’s attack on the 19-year-old woman, who was hospitalized in critical condition.

“From what we know, this attack was sudden and completely unprovoked,” D.C. Police Chief Cathy L. Lanier said.

Davis comes from a family of prominent athletes. Two of his brothers play in the National Football League: Vernon Davis, a tight end for the San Francisco 49ers, and Vontae Davis, a cornerback with the Miami Dolphins. Like Michael Davis, they grew up in the Emerson Street rowhouse with their grandmother.

“Just got a disturbing call from back home in washington dc,” a message on Vernon Davis’s Twitter account said Friday. “A good day gone bad! Hopefully he wasn’t involved.”

Police said Michael Davis is under investigation in connection with the three other assaults, which occurred Tuesday and Wednesday. As of Friday evening, he had not been charged.

Two of the victims in those cases survived with severe head injuries. The other, a 66-year-old Denver man, was found dead in an alley about a block from the rowhouse where Davis lives, in the 900 block of Emerson Street NW.

“At this time, there is no apparent motive,” Lt. Robert Alder of D.C. police’s homicide unit said Friday, standing outside the Davis home as investigators searched inside.

“I see him all the time,” a neighbor, Cathy Meads, said of Davis. She stood on her front porch, watching evidence technicians enter and leave next door, on a street crowded with police cars and news trucks. “Wouldn’t have thought nothing like this would happen,” Meads said, describing Davis as “a nice young man.”

“I’d walk up the street, don’t worry at all about him hitting me on the head,” she said.

After police finished their work at the rowhouse Friday, a clutch of reporters and TV camera operators crowded onto the front porch, one of them ringing the doorbell repeatedly. No one responded. A trash bag was taped over the door window inside. Phone calls seeking comment on the arrest went unanswered at the home.

Alder declined to discuss Davis’s mental condition in the past or at the time of his arrest. Davis was scheduled to make his initial appearance Saturday in D.C. Superior Court. Meanwhile, police said, detectives are continuing to seek evidence — including DNA from the hammer — that could tie him to the earlier assaults.

In a July 27, 2009, entry on his blog, Vernon Davis told fans that he and his brother Vontae had taken part in a magazine photo session at the University of Maryland campus in College Park, where Vernon Davis had been a football star.

“The highlight of the day came when my little brother Michael stopped by and ran routes with Vontae and I. . . . Man, Michael is more talented than both Vontae and I. I just hope he works hard and sticks with it so he can join us in the NFL one day.”

Thursday’s assault occurred a little over four hours after Lanier warned the public at a news conference that a serial attacker might be at work in the Petworth area, citing the three “sudden” and “unprovoked” attacks.

Police withheld the names of the victims who survived, publicly identifying only the man who was found dead Tuesday night.

Gary Dederichs, 66, had retired as a nurse after working at the Swedish Medical Center in Denver. Friends and relatives said he enjoyed traveling. In planning trips, they said, he used a Web site that matches tourists with people who have rooms to rent.

“Retired, neat, quiet gentleman,” was how the unmarried Dederichs described himself on the site. Police said he had been staying with a homeowner in Petworth in recent weeks while he explored the nation’s capital.

In his neighborhood near the University of Denver campus, he volunteered to run errands for elderly people and help them with household tasks as part of a program that enables them to continue living in their homes. “The folks he helped are grief-stricken,” said Paul Ramsey, executive director of A Little Help.

“He had a gentle and protective way with me,” said Beatrice Adams, 83, who uses a walker. “Gary insisted I wait until he came to the door to get me and walk me every step to the car real carefully. And the same when we got to the doctor.”

Police said the other victims were a 53-year-old man, who was found Wednesday morning on Georgia Avenue NW, bleeding from a serious head injury, and a 37-year-old woman, who suffered a severe head wound Wednesday night in an attack on Ninth Street NW.

Police said Thursday night’s foot chase began on Gallatin Street and proceeded through yards and over fences. At one point, they said, Davis hid behind a portable toilet, then started running again, taking off clothing as he fled. When they caught him, police said, Davis was on the porch of a house on nearby Illinois Avenue NW.

Although the four attacks have not been conclusively linked, Lanier said, “I feel very much more confident now that this person is in custody.”

Staff writer Maggie Fazeli Fard and researcher Jennifer Jenkins contributed to this report.