BOSTON (CBS/AP) – Sections of Boston that lost power because of a smoky electrical transformer fire are slowly recovering but problems are expected to persist for most of the day.

Photos: Back Bay Blackout

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About 4,000 customers were still without power late Wednesday, according to NStar, down from a high of about 21,000 Tuesday night.

WBZ NewsRadio 1030’s Carl Stevens reports

The fire, in a 115,000-volt transformer, was first reported at about 6:30 p.m. Tuesday near a garage behind the Back Bay Hilton Hotel. That hotel and the nearby Sheraton were evacuated as a precaution. The fire greatly diminished after the power was shut off.

There is still no word yet on a cause.

“Our focus now is getting customers up and running, and after that we’ll look for the cause,” NStar spokeswoman Caroline Allen said.

It’s not clear how old the transformer was.  NStar said the station it was in was built in 1973.  A second transformer was also damaged in the fire, but authorities believe it can be repaired.

A source tells WBZ-TV’s Jonathan Elias it will take days, not hours for the substation to be repaired.

6 portable generators on flatbeds from New York arrived at NStar's site Wednesday morning (Photo by Chris Gobeille)

Once the second transformer is fixed, officials hope to wean the area off the generators and return it to its normal power supply.

Two miles of cable are now running through city streets, similar to extension cords, supplying power to some neighborhoods.

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Many businesses, as well as the Boston Public Library, remained closed. People showed up for work Wednesday and were told to go home.

Some roads also remained blocked off to traffic and since the Prudential station on the Green Line does not have full power, fares will be collected on the trolley.

State Police say the Prudential ramp, Exit 22 on the Mass Pike, will be shut down for approximately two days to accommodate two generators.

Police will maintain a visible presence in the affected neighborhoods until things are back to normal, Police Commissioner Edward Davis said.

There were no reports of looting or other problems, he said.

“It’s a real testament to the people of the city,” he said. “… Even the driving has been respectful.”

The city is sending inspectors to restaurants in the affected area to make sure no spoiled food is being served, Mayor Thomas Menino said.

The fire plunged a huge swath of the city, from the Back Bay, to Chinatown, to Kenmore Square into darkness. Skyscrapers, normally lit up even when no workers are inside, stood eerily dark overnight. Hotels, bars and some homes were evacuated.

There was some concern that the heavy black smoke that billowed across the city the first minutes of the fire was toxic. Authorities determined it was not.

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