The first victim on a violent Monday night in the District was a 16-year-old who was walking down Florida Avenue NE at 8:15 p.m. when, he told police, someone in a ski mask hopped out of a green sport-utility vehicle and fired a single shot.

Over the next few hours, four more teens were shot, including a 17-year-old on M Street NW in Georgetown at the height of one of the city’s largest Halloween celebrations. None of the shootings was fatal.

Police also broke up a brawl at the Foggy Bottom Metro station. Then, in the wee hours of Tuesday morning, a 44-year-old man heard gunshots in the 4200 block of Barnaby Road SE. He started to run and was hit in the leg.

In one night, six people were shot in varied sections of the District. Police said there was no pattern to the violence. The victims were shot in Petworth and Trinidad, two up-and-coming neighborhoods; in Georgetown, one of the city’s wealthiest sections; and in Washington Highlands in Southeast, one of the poorest.

“In the 21 years that I’ve been [in the District], October is always one of our more challenging months,” Police Chief Cathy L. Lanier said.

But, she said, Monday night’s shootings were not part of a trend. Violent crime is down this year compared with last year, Lanier said: Homicides, at 95, are down from 110 at this time last year, and there have been 418 assaults with a dangerous weapon this year, compared with 510 in 2010.

Officers had announced one arrest in the latest shootings: a 24-year-old man they chased down near the scene of the Georgetown shooting. He was charged with carrying a pistol without a license. Officers would not say whether he was involved in the shooting. The victim, who was apparently shot in the head, was in critical condition.

As she often does on nights when lots of people plan to be out, Lanier had ordered extra officers on the streets. Monday night started out relatively quiet, she said, with smaller-than-expected crowds at some traditionally popular Halloween gathering places — U Street NW, Adams Morgan, Chinatown and Georgetown.

Then, as trick-or-treaters headed home, police dispatchers started what would be a busy night. The first call was to Florida Avenue near Benning Road NE, where the victim told officers to seek a masked gunman in an SUV, possibly a Cherokee. Investigators were considering other explanations based on witness accounts, police sources said.

At 9:04 p.m., officers had another mystery. Two teens, 16 and 19, were hanging out at Georgia Avenue and Decatur Street NW when they heard gunshots and started running. Both were hit in the leg.

At 10:08 p.m., just blocks from the Decatur Street shooting, an 18-year-old woman was walking down the 800 block of Crittenden Street NW when she heard the crackle of gunfire and took off running. She was hit in the ankle, police said.

Because of the proximity of those two shootings, police were trying to determine whether they were related. D.C. Council member Muriel Bowser (D-Ward 4) said the Crittenden Street victim had been at the scene of the Georgia Avenue shooting.

In Petworth on Tuesday, Rufus Boyd, who lives in a rowhouse near the shootings, said the neighborhood was crowded with children on Halloween night.

“There were some little kids running back and forth, with their parents. You know, trick-or-treating. It was a nice-looking setting,” he said. “But there was some crazy misfortune.”

He was inside when he heard shots — and stayed there.

“After that, I was just hoping I’d hear an ambulance,” he said. “You don’t want to see anybody get hurt.”

At 10:40 p.m., Metro Transit Police were called to the Foggy Bottom Station, where 30 to 40 teens had started fighting. A few dozen officers broke up the brawl, said Dan Stessel, a Metro spokesman. No one was arrested.

Nine minutes later, officers were racing to nearby Georgetown, where M Street and many surrounding side streets were closed to give hundreds of trick-or-treaters and bar hoppers free rein. The 17-year-old was shot in front of Don Lobo’s Mexican Grill, across the street from the Four Seasons hotel.

Investigators think two groups of more than a dozen teens from different neighborhoods met by chance, law enforcement sources said. They exchanged words, which led to gunfire, and the victim fell immediately, the sources said. Police think three guns were fired in the incident.

Council member Jack Evans (D-Ward 2) said the increased police presence in Georgetown on Halloween has worked well over the past 10 years.

“This is the first time we had any real bad incidents,” Evans said.

Franklin Ramirez, manager of Don Lobo’s, said that at the time of the shooting, the restaurant was closed and that he was waiting at a table near the front window for a friend; they planned to walk around the neighborhood.

Ramirez said he walked to the back to go to the bathroom. “Then I heard three shots.” Stunned, he walked back out into the restaurant and, through the window, saw people scattering. A group stood over a man who was on the ground and bleeding from the head, Ramirez said.

“I’ve seen things like this in movies, but never in real life,” he said Tuesday.

Police recovered a shell casing that came through the window of the restaurant’s front door. Part of the glass door was still shattered Tuesday.

“This was a night where everybody comes out to have fun and enjoy themselves, and for this thing to happen,” Ramirez said. “I want to know why someone would do this.”

Violent crime was up last month in the District compared with October 2010. At a news conference Tuesday, Lanier said that Mayor Vincent C. Gray (D) had tried to explain how a night of revelry could turn so violent in so many sections of the city.

“Last night, with all these shootings that occurred, [is] a symptom of the bigger problem,” Lanier said, which is an “uptick” in the number of guns flowing into the city.

In most of the shootings Monday night, the violence appeared random, Lanier said. The victims were just out walking, “friends visiting friends,” when the violence erupted, she said.

The last shooting occurred at 1:35 a.m. Tuesday. An Oxon Hill man was in the 4200 block of Barnaby Road SE when he was shot in the leg.

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Staff writers Sylvia Carignan, Tim Craig and Clarence Williams contributed to this report.