Investigators are looking at disgruntled former Goldman Sachs workers as possible suspects in their efforts to find the person who mailed dozens of letters threatening the investment firm last month, federal officials said Tuesday.

The letters, handwritten in red ink on lined, loose leaf paper, dealt a terse threat: “Hundreds will die. We are inside. You cannot stop us,” authorities said.

The warning was signed, “A.Q.U.S.A.”

Investigators have compiled a list of potential suspects, considered “people of interest,” which includes former employees, a spokesman for the U.S. Postal Inspection Service, Tom Boyle, said.

“We are investigating certain individuals either to remove them from the list or to gather more information on them,” Mr. Boyle said.

He said that no arrest is imminent.

The FBI was not placing high credibility to the threat, a federal law enforcement official told The Associated Press.

“Investigators are working on the theory that the individual was trying to target negative publicity against Goldman Sachs,” said the official, who spoke on the condition of anonymity because he was not authorized to discuss the case with the media.

Authorities have pinpointed two postal collection areas where the letters were dropped into sidewalk mailboxes in Queens and Manhattan. The latter passed through the Morgan postal facility, on Ninth Avenue between 30th and 31st streets, according to Mr. Boyle.

The letters, postmarked late June, were received by 31 newspapers in states nationwide, including Arizona, North Dakota, Texas, Ohio and New Jersey. The newspapers notified the FBI or local law enforcement and several made the incident public by publishing it last week.

The letters are being analyzed at the FBI criminal laboratory in Washington and at the postal service lab in Dulles, Va. Goldman Sachs is cooperating fully with law enforcement authorities, a company spokesman, Michael DuVally, said. He declined to comment further on the investigation.

Goldman Sachs, based in New York, has offices in London, Frankfurt, Tokyo, Hong Kong and other cities. The firm has 28,000 employees worldwide, including about 3,000 people who work in its 44-story tower in Jersey City, N.J.