A Pakistani man arrested by the FBI in its investigation of the Sept. 11 terrorist attacks was found dead in his jail cell, authorities said.

The 55-year-old man, whose identity was not immediately released, was found dead Tuesday morning at the Hudson County Jail in Kearny. An autopsy was planned to determine the cause of death.

A doctor who examined the man's body said a nasal swab showed no signs of anthrax, but further tests were planned, county spokesman Jacob Delemos said.

The man was arrested Sept. 19 as part of the investigation into the terrorist attacks, a government source who insisted on anonymity told The Associated Press on Wednesday.

He was being held on immigration charges, authorities said. Federal immigration officials didn't immediately return calls for comment.

The man had complained recently of pain in his gums, saw a dentist and was given tetracycline, an antibiotic that can be used to treat anthrax, Delemos said.

No prisoners were quarantined, he said.

Special Agent Sandra Carroll, a spokeswoman for the FBI's Newark office, would not say how the man figured into the investigation.

"We never filed criminal charges against the individual," she said. "I can't say whether we considered him an integral part of the Sept. 11 investigation or not."

After the death, hazardous materials experts were sent to the jail as a precaution, but they left without performing any tests or cleanup, Deputy Fire Director Jose Cruz said.