“That’s very, very worrisome,” said D.C. Council member Mary M. Cheh (D-Ward 3), who represents the neighborhoods where the incidents occurred. “People are starting to feel insecure.” (Sarah L. Voisin/The Washington Post)

The college-age partygoers lingered on the grass outside a house in Northwest Washington after 1 a.m. Monday, the thump of the bass from inside the residence easily audible. They were caught by surprise when the three men crept up from behind and yelled, “Nobody move.”

One assailant punched a victim in the face, sending him tumbling to the ground, according to a D.C. police report. Another placed a victim in a chokehold, then pressed the cold barrel of a black handgun against the side of his head and said, “Give me everything you got.” The robbers made off with a white iPhone 6, cash, a debit card and a Swatch timepiece.

Minutes later, police believe, the same assailants attempted to rob two men about a mile away in front of a sleek, modern condominium building located above the Tenleytown Metro station and across from a District elementary school. But the robbers fled before finishing the job, jumping into a four-door Nissan sedan and driving away.

The two incidents, occurring between 1:18 a.m. and 1:34 a.m. Monday, continued a rash of brazen crimes in what is usually a placid part of the District.

“That’s very, very worrisome,” said D.C. Council member Mary M. Cheh (D-Ward 3), who represents the neighborhoods where the incidents occurred. “People are starting to feel insecure.”

Cheh said police are describing the two crimes as connected and probably committed by the same perpetrators. The incidents resemble a string of holdups that transpired in late July, some in the same area of the city. But Cheh said that police do not believe the two sets of robberies are linked.

Natalia Bonansea said she was at home in the 3700 block of Harrison Street NW on Sunday evening when she heard the house next to hers hosting a loud party. She said young people spilled onto the front lawn, where red cups and empty beer cans were still strewn about at midday Monday.

“It was a huge party, like a crazy amount of people,” Bonansea said.

According to the police report, four men and one woman were outside the house when the three robbers showed up brandishing pistols.

“They would have been easy targets, being drunk college kids,” said Bonansea, who noted that the leafy neighborhood is considered one of the safer places to live in the city. “This is the worst I’ve heard,” she said.

Less than 15 minutes later, three apparent would-be robbers showed up at 4101 Albemarle St. NW, a condominium complex at the bustling intersection with Wisconsin Avenue.

Cheh, who spoke to police about the incidents, said investigators believe it was the same three men who had been on Harrison Street a short time earlier.

The trio approached two men on the sidewalk, and attempted to rob them at gunpoint, the police report says. One suspect, in a white muscle shirt and a black bandanna face mask, held a black handgun. The report said, “The suspects were confronted [by one of the victims], and left the scene without taking any property.”

Caleb Skeath, a lawyer and American University graduate who lives in the condo building, said he was stunned that such an incident occurred in front of the complex.

“Having it happen right out front is a little unsettling,” Skeath said. “It’s definitely a lot safer than other areas of D.C. But stuff happens.”

The earlier series of robberies, on July 28, began in the Chevy Chase neighborhood, near the Maryland line, and continued in Tenleytown and across the city into Northeast. Those assailants stole $1,700 in cash and several cellphones during at least six armed holdups, police say.

Cheh said that police are increasing patrols in the areas where the recent crimes occurred, in hopes of calming jittery residents. On Monday afternoon, a patrol car was stationed around the corner from the second robbery location.

“Psychologically, it’s very, very disturbing,” Cheh said.

On Monday night, D.C. police also were investigating multiple armed robberies elsewhere in the city, including two in Northeast in which assailants were seen leaving in a blue Jeep Cherokee. In one of those incidents, police said suspects were wearing ski masks and no shirts.

Aaron C. Davis contributed to this report.