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The FBI said Monday that several more suspects in the World Trade Center explosion could be arrested this week in a widening probe into the $1 billion bombing.

James Fox, who heads the FBI’s New York office, said that “as many as 15 to 20” people may have been involved in the terrorist conspiracy. “I would say three or four would be a minimum,” he said.

An FBI spokesman said two more apartments were searched over the weekend not far from the Jersey City residence of Mohammed Salameh, 25, a Palestinian with Jordanian citizenship arrested last week in connection with the Feb. 26 bombing.

But the FBI declined to elaborate further on other suspects in the case or what may have been seized in the Jersey City residences. Two local newspapers reported that agents removed three alarm clocks from the premises, presumably to compare them with any parts of a timing device that might be found at the site of the explosion.

“We could have additional search warrants, and possibly arrest warrants, this week,” Fox said in a television interview. He said the investigation would continue “for months, at least.”

At the World Trade Center, investigators and construction workers used “spider” scaffolding, similar to the lifting devices used by high-rise window washers, to inch their way to the bottom of the bomb crater six stories under the streets of lower Manhattan.

Their work was hampered by the dangers created by “tremendous chunks of concrete” dangling from the steel rods that reinforced the flooring of the underground parking facility, said Richard DeFillippo, an FBI spokesman.

“For those who are working there, it’s a very slow, arduous procedure, but they’re making progress,” DeFillippo said.

Searchers still have not been able to locate a maintenance worker who has been listed as missing in the garage. Traces of blood were found in the garage, but investigators could not identify its source, said a spokesman for the New York-New Jersey Port Authority, which operates the building.

Port Authority officials said Monday that they hope to reopen the 110-story twin towers, the world’s second-tallest buildings, by April 1.