NEW YORK (AP) – A mysterious electrical problem that swept through northwest Queens this week persisted for the fourth straight day Thursday, leaving nearly 2,000 customers powerless and thousands with limited voltage.

The blackouts have been occurring since Monday evening in a handful of neighborhoods and have taken place over a span that included two of the hottest days of the year. The problems were at their worst Wednesday, when 10 of the 22 feeder cables that supply the area with power were down simultaneously.

Things improved on Thursday, though not quickly enough for many.

Subway service to Queens was restored to near normal levels by 4:45 a.m., a day after the number of trains was cut sharply because of inadequate power to the system’s signal network.

LaGuardia Airport was back to normal after experiencing flight cancellations for most of Tuesday because of blackouts in two terminals.

Still, hundreds of businesses sat idle. Restaurants threw away spoiled food. The city’s jail complex on Rikers Island operated on backup generators. Thousands of homes had enough electricity for lights but too little voltage to run some appliances.

“This is outrageous,” said City Councilman Peter F. Vallone Jr. “When is this going to be fixed? If it’s going to be days, they should tell people it is going to be days.”

It was unclear how many people were affected because a utility customer could be a single-family home or a big apartment building.

Consolidated Edison spokesman Chris Olert said the power company was making every effort to get the system running but couldn’t estimate when that might happen. He also said the company still didn’t know why things went so wrong.

“Chances are fair, but not firm, that it was heat related, but right now that is just a hypothesis, not fact,” he said.

A series of heavy-duty circuits that supply the area began to fail Monday evening, just hours after the sweltering state set a record for electricity use.

More circuits failed on Tuesday and more again on Wednesday, even after the city’s heat wave ended and demand for power plummeted.

Just why heat would have triggered a problem in Queens, but not elsewhere, was unclear.

“It was hot in every borough, and the same thing hasn’t happened elsewhere,” Olert said.

Some possibilities, he said, included the age of the feeder cables, physical damage to them or some type of cascading effect, in which each failure put additional strain on the surviving circuits.

Mayor Michael Bloomberg said at an evening news conference in Queens that he was satisfied that Con Edison was working hard to fix the problem, at times going building by building to check current.

He added, though, that he had demanded that the utility investigate the outages and deliver a report on their cause within two weeks.

AP-ES-07-20-06 1906EDT