Fire fighters blocked off River Street at its intersection with Memorial Drive yesterday morning as they investigated the source of smoke pouring out from three utility entry ports.

As a police officer re-routed traffic coming down Memorial Drive and across the River Street Bridge, several fire trucks and emergency vehicles parked in and around the Shell and Mobil stations on either side of the street.

According to Bob Cuggino, supervisor of the construction project which is preparing for a new convenience store at the Shell station, his crew accidentally cut an unusually shallow service cable.

Cuggino said that in addition to being too close to the surface, the old pipe which housed the wire used illegal aluminum coupling.

"If he had yanked that thing he could have gotten electrocuted on that machine," said Cuggino, gesturing towards a construction worker and his backhoe. "That's why we think it was already broken, from age maybe, and he just knocked it a little and the wire hit the pipe."

"We noticed the smoke, ran to the phone and called the fire department," said Cuggino. "They came and did a great job."

But fire department officials suggested that the backhoe may not have been to blame.

"The construction [work on the Shell station] has absolutely nothing to do with what's going on over there," said Dennis J. Marogioglio, an aide to the deputy chief of the Cambridge Fire Department.

Cambridge Deputy Fire Chief Gerald R. Reardon suggested that the problem could have been caused by the heat and the summer's greatly increased demand on electrical power.

But repair workers from Cambridge Electrical said the accident most likely was caused by the construction work.

"We just had a [construction worker] cut the secondary service to that gas station over there," said Line Supervisor Larry L. Anthony, pointing at the Shell station. "He must have hit it with a backhoe."

Anthony said the electricians had shut down power to a small portion of the surrounding area--"maybe 10 to 12 residences"--as well as the nearby traffic lights.

"We shut down the transformer," Anthony said. "We're trying to isolate the problem then we can pick everybody back up except [the Shell station]."

Cuggino said the service was about to be replaced in any case.

"We were going to cancel it out tomorrow anyway and bring a new service in," he said.

Fire department officials said the cut line posed little danger.

"It's an electrical problem in a manhole--that's all," said Reardon. "We just came down to make sure there wasn't smoke in any of these buildings."

"It happens," said Cuggino. "Nobody got hurt, that's the important thing."