Investigators have compiled a list of potential suspects that includes former employees at Goldman Sachs (GS) as they try to find the person, or persons, 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."

The list of "people of interest," includes disgruntled former employees, said Tom Boyle, a spokesman for the U.S. Postal Service.

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

He said that no arrest is imminent.

The threat of violence was not considered highly credible by the FBI, 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 letters passed through the Morgan postal facility, on Ninth Avenue between 30th and 31st streets, according to Boyle.

Postmarked late June, the letters were sent to at least 31 newspapers in several states, including Arizona, North Dakota, Texas, Ohio and New Jersey. The newspapers notified the FBI or local law enforcement.

The letters are being analyzed at the FBI criminal laboratory in Washington and at the postal service lab in Dulles, Va. Goldman Sachs has been working with investigators.

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.