Danielle Lake had heard, and heeded, the warnings about taking her cellphone out in public. But after three buses sped by her Georgetown stop Tuesday night, none of them hers, the 26-year-old just had to check the online bus schedule.

It was the last time she saw her iPhone 4S.

“A guy riding a bicycle just snatched the phone out of my hand,” said Lake, who agreed to be identified.

D.C. police said that the incident appears to be the last of three similar snatch and grabs that occurred within 90 minutes on the crowded streets of Georgetown and neighboring Burleith in Northwest Washington. In each case, police said, the thief was riding a dark-colored bicycle.

The other thefts occurred on Wisconsin Avenue and on R Street, where the victim was a 15-year-old boy who police said ran after the bicyclist and managed to grab the back tire, flipping the bike. The boy’s iPhone fell to the pavement, and he got it back. Police said the thief got away.

Authorities have repeatedly spoken about the dangers of using cellphones in public. In the District, phones were taken in about half of the more than 3,400 armed and unarmed robberies this year. That is double the number taken six years ago, D.C. Police Chief Cathy L. Lanier has said. The number of robberies across the city are on par with last year; they are down in the police patrol area that includes Georgetown.

“As you know, the scope of the problem is massive,” Lanier told a D.C. Council committee in May. “In cities around the country, the robbery and theft of small personal electronics is driving an increase in crime as thieves target unattended small electronics in cars, pockets, purses and on tabletops.”

Lake, who moved to the District from North Carolina two years ago to pursue a career in architecture, said she is well aware of the problem of cellphone thefts. She had just read an article about it last week and said she is “extra vigilant.”

On Tuesday, she left her office on Thomas Jefferson Street and walked to M Street to catch a bus home. It was about 6:45 p.m., but Lake said there was still daylight, the street was crowded and “everybody had their phone out.”

She said the bicyclist was riding on the sidewalk and grabbed her phone before she noticed him. “It was just the luck of the draw,” Lake said of being targeted. She said she chased the bicyclist and screamed, “Stop that bike! Stop that bike!” but that no one did. She returned to her office to cancel her phone service and call police. Lake said she tried to track the phone but that the thief had turned it off.

The first robbery was about 5:30 p.m. in the 1600 block of Wisconsin Avenue. The victim told police that she was holding her phone and wallet in her hand and that both were stolen as the bicyclist pedaled by.

About an hour later, police said, the bicyclist struck in the 3500 block of R Street. Police said that Lake was targeted less than 15 minutes later.