The gunmen struck fast as they held up construction workers at three sites in the same block in Northeast Washington. Police said officers were taking a report at one location when "another call came out with another robbery" around the corner.

Authorities have not made any arrests and are looking for two or three men in three armed robberies of construction workers that occurred around lunchtime Monday in the District's Woodridge neighborhood, adjacent to Brookland along Bladensburg Road.

The attacks prompted D.C. police on Friday to issue safety tips to construction workers, such as to avoid working outside alone and to keep valuables locked up and hidden. Police say homeowners having work done on their houses can request extra patrols.

In August, a 34-year-old construction worker, Balmore Mauricio Fuentes, was fatally shot during a robbery at a house he was renovating in the Trinidad neighborhood of Northeast Washington. A 14-year-old boy was charged as a juvenile with first-degree murder.

Monday's robberies began shortly after 11:30 a.m. at a residence in the 2800 block of 30th St. NE. Police said in a report that three men walked into a house under construction and that one held a gun to a worker's back. Police said the gunman ordered the victim to call another worker in off the roof. The two were robbed of credit cards and $600.

A few minutes later, police said, the assailants fled to another location around the block. There, in the 2800 block of Bladensburg Rd. NE, police said, the men robbed four workers who were on their lunch break from doing renovation work at the New Canaan Baptist Church.

A police report said one of the assailants rifled through the men's pockets and took money and wallets. The gunman fired one shot toward them and ran toward Yost Place NE. No one was struck.

Police said that while officers were interviewing the victims from Bladensburg Road, a call came in about another robbery, this one in the 3000 block of Yost Place NE. Police responded, but the assailants already had fled.

Police said that in this case, a construction worker was accosted by a gunman while outside. He was ordered inside a house where two other workers were, and all three were robbed, police said. Again, money and bank cards were taken.

Police said they are investigating whether a fourth armed robbery two days later in Southeast is connected to the others.

On Wednesday, police said, two construction workers were robbed at a house in the 3600 block of Camden Street SE, near Naylor Gardens and about eight miles from Woodridge. Police said two men, both wearing masks and one armed with a gun, robbed the pair as they worked on a house. It occurred about 10:30 a.m.

Police said money, credit cards and an iPhone were taken.