A masked man armed with an assault rifle opened fire on a residential street in Northeast Washington yesterday morning, killing one man and wounding three others. The slaying was the latest in a week-long spike of deadly violence in the city.

The homicide, the ninth in seven days, pushed the District's total for the year to 113, one more than recorded during the same period in 2004.

D.C. police said they believe the homicides stem from typical increases in violence during summer. July and August usually are among the deadliest months, statistics show.

Police officials reacted to the slayings by authorizing more overtime for patrol officers and homicide detectives. Commanders will also shuffle schedules to put more police on the street during weekends, officials said.

"It seems to be a summertime phenomenon," said D.C. Police Chief Charles H. Ramsey. "There is this rash all of a sudden. There is no explaining it.

"It seems to be part of the overall climate we're in now."

Despite the recent spike, the District remains on pace to record fewer than 200 homicides for the second year in a row. It would be the first time the District has tallied so few homicides in consecutive years since the mid-1980s.

In yesterday's quadruple shooting, the gunman opened fire on a white box truck carrying five men about 8:10 a.m. The gunfire shattered the morning quiet in the 5700 block of Sixth Street, police and residents said.

Several people said they heard what sounded like machine-gun fire. Police recovered at least 12 shell casings at the scene, authorities said.

Police said the gunman might have lured the victims to the scene and ambushed them. He fired first at two men in the truck's cargo area and then turned the rifle on the cab, police said.

The truck drove onto Madison Street, where one of the victims fell from the cargo space onto the street. He was pronounced dead at the scene.

The driver then went around the corner and stopped at a dry cleaning business at Riggs Road and Chillum Place, police said.

One victim ran inside the store, and the owner called 911, police said. The gunman fled in a gray or silver car.

Three victims -- two 17-year-olds and a 40-year-old -- were taken to hospitals with serious injuries. Police did not release their names because the men are witnesses. All were listed in stable condition last night, police said.

The truck had been owned by Son's Quality Food Co. but had been given as a charitable gift to a nonprofit group, said the company's owner, Chong Son.

Residents expressed shock, saying their neighborhood -- which has a shopping mall, rows of well-kept homes and an elementary school -- has a persistent drug problem but rarely sees such violence.

"It's so vicious to shoot somebody in broad daylight like that," said Michael Hooper, 50, who lives near the site of the shootings.

Claudia Sullivan, 60, said her husband heard rapid gunfire and then she heard sirens and a police helicopter circling the neighborhood.

"Nothing like this happens here," Sullivan said. "It's terrifying. It's not good. It's terrible and horrible."

D.C. Council member Adrian M. Fenty (D-Ward 4), who represents the area, said the shooting "was one of the most violent crimes to ever happen in that area."

"One resident told me it sounded like Iraq," Fenty said. "It was so loud and repetitive that he didn't even think it was gunshots. . . . This guy was brazen. He had no regard for anything."

The killing was the second in the District in a three-hour span. An unidentified man was found shot about 5:30 a.m. at South Capitol and Danbury streets SW.

In another shooting, about 11:30 p.m. Wednesday, three people were wounded in the 1500 block of Kenilworth Avenue NE, police said. A 20-year-old man was hit in the foot and thigh, and a 15-year-old boy was wounded in the right leg. Another 20-year-old man was shot in the head and was described as being in grave condition at a hospital, authorities said.

A gunman opened fire about 8:10 a.m. on a box truck carrying five men, police said, in an area that residents said is unaccustomed to such violence. Nine people have been killed in the District in the past week.Police look for clues near Madison Street NE. Officials said the recent rise in killings stems from a typical increase in violence during the summer.