A routine screening yesterday at a U.S. Customs Service center off North Capitol Street in the District uncovered a cache of plastic explosives sent by an American citizen now in the Persian Gulf, federal officials said.

The package, which contained about four pounds of an explosive known as C-4, was uncovered about 1:30 p.m. by a National Guard detail that routinely examines overseas mail at the facility in the unit block of M Street NE, near the city's main post office.

The FBI, one of several agencies called to the mail center, said the package was not part of any "terrorist-type activity," said bureau spokesman James Mull.

He added that the investigation was being handled entirely by the Customs Service.

The clay-like substance, often used in pipe bombs, needs an explosive device to detonate. Officials said the package was stable and that there was no threat of an explosion.

Under the heightened security prompted by the Persian Gulf War and the threat of terrorist activity in the United States, yesterday's discovery caused an immediate flurry of activity and, at least initially, a degree of secrecy.

Technicians from the D.C. bomb squad, which was called to the mail center, said at the scene that nothing had been found.

But customs officials, without providing many details, later confirmed the explosives had been found.

The amount of damage that can be caused by C-4, or Composition 4, depends on the quantity and where it is placed.

Ted Postol, professor of science, technology and national security policy at the Massachusetts Institute of Technology, estimated that four pounds of the explosive could do substantial damage. Just how extensive depends on where it is placed.

In a building, that quantity of C-4 could knock down walls at a range of 40 to 50 feet. In an open-air setting, it could blow out windows in buildings within 100 feet. In some instances, the amount could be even deadlier.

"You wouldn't want it to go off in a plane," Postol said. "That would bring the plane down."

Steven Devaugh, director of the Customs Service's Office of Commercial Fraud Enforcement, said investigators are working with other federal agencies to arrest the person who mailed the package.

The parcel was en route to a location in the western United States, and the U.S. Postal Service is one of the agencies investigating. It is a federal felony to import explosives, but it was unclear yesterday what the penalties would be upon conviction.

Although fears of terrorism have prompted more vigilance by customs agents, officials said yesterday's discovery was not a result of a recent change in the monitoring of packages.

The National Guard has been helping the Customs Service search for narcotics illegally mailed into the country for some time.

The program in the M Street facility is one of many across the country, spokesman Bill Anthony said.