The FBI believes the group that claimed responsibility for an explosion that damaged the Capitol Monday night has been involved, either directly or through affiliated groups, in a series of bombings of federal or corporate offices in New York and Washington over the last year, according to informed sources.

A tape recorded message was telephoned to The Washington Post moments before the explosion at 11 p.m. and said that the action was being taken by the Armed Resistance Unit in support of "all nations' struggle" against U.S. military aggression in Grenada and Lebanon. A second call, also recorded, warned the Capitol switchboard that a bomb was about to go off.

Moments later, an explosion left piles of rubble just outside the doors to the Senate chamber, blowing out a wall partition and windows, ripping through the Republican Cloakroom, and damaging several works of art on the second floor. The bomb appeared to have been placed on or under a window well seat in a corridor leading to the Senate chamber.

Congressional aides said the blast, while it apparently caused no structural damage, probably will cost at least $1 million to repair.

Security was immediately tightened at the Capitol and other government buildings yesterday. Several federal office buildings and Metro's Capitol South subway station were closed temporarily after numerous bomb threats were received during the day. The D.C. police warned commanders that terrorist groups could strike again and ordered special surveillance of federal and District government buildings, courthouses, embassies and police facilities.

In a news conference yesterday morning, an outraged Senate Minority Leader Robert C. Byrd (D-W.Va.), whose office doors were blown off in the incident, called the explosion "an offense against all the people."

"It was like a shrapnel explosion," Senate Majority Leader Howard H. Baker Jr. (R-Tenn.) said. "It really could have been bad; there could have been a loss of life if the Senate had been in session."

Sen. Jeremiah Denton (R-Ala), chairman of a security and terrorism subcommittee of the Senate Judiciary Committee, described the bombing as "an attack that strikes at the heart of our constitutional democracy."

"It takes incidents such as the attack on our marines in Lebanon or a bomb going off in the Capitol to obtain the attention that could have and should have been accorded to the problem earlier," said Denton, who sharply criticized the news media for not giving sufficient coverage to the earlier findings of his subcommittee.

The bomb appeared to have been placed near the Mansfield Room, across the hall from the Republican Cloakroom and diagonally across from the minority leader's office.

It caused major damage to arches, and the walls outside and inside the Republican Cloakroom were pockmarked with fist-sized holes. Glass in the wall separating the hallway from the cloakroom was shattered.

The explosion ripped a 19th-century oil portrait of Sen. Daniel Webster from its gilt frame and shredded a portrait of John C. Calhoun, Capitol officials said. Four other paintings were slightly damaged and were removed for examination.

Other prized furnishings also were damaged. A grandfather clock that has stood outside the Senate chamber since 1859 was stopped by the blast, which also blackened one eye of a marble bust of Theodore Roosevelt. Glazed tiles installed in the 1850s by English craftsmen were torn from the floor and propelled through the window of the Republican Cloakroom. Woodwork and gold molding outside the Mansfield Room were ripped apart in the explosion.

The FBI, which is investigating the incident along with the District police, Capitol police and Secret Service, is understood to be reviewing video tapes taken by surveillance cameras hidden in Capitol alcoves in an effort to determine the identity of the person or persons responsible.

At a news conference yesterday, Theodore M. Gardner, the FBI's special agent in charge of the Washington Field Office, said that dynamite appears to have been used.

"It was a high-explosive device with delayed timing," Gardner said, disclosing that the Armed Resistance Unit is the same group that claimed responsibility for a bomb blast last April at the National War College building at Fort McNair in Southwest Washington.

In that incident, as in Monday's, no one was injured, the explosive material was set off with a timing device, and the blast was preceded by tape recorded messages played during telephone calls. The messages referred to U.S. imperialism, American bases, and Guatemala.

Yesterday, National Public Radio received in the mail a typewritten "communique" from the Armed Resistance Unit saying, "Tonight we bombed the U.S. Capitol building."

The letter continues: "We attacked the U.S. government to retaliate against imperialist aggression that that has sent the Marines, the CIA, and the Army to invade sovereign nations, to trample and lay waste the lives and rights of the peoples of Grenada, Lebanon, El Salvador, and Nicaragua, to carry out imperialism's need to dominate, oppress, and exploit."

Saying the group consciously aimed its attack at American institutions as opposed to individuals, the letter refers approvingly to attempts by nationalists to create an independent Puerto Rican nation and calls for "Victory to the FMLN/FDR" and support for the Palestine Liberation Organization (PLO).

FMLN and FDR are two groups among interrelated organizations that have claimed responsibility for the bombings that have plagued institutions in Washington and New York, the FBI believes. Several receive backing and money from the Cuban government, according to information received by the bureau from U.S. intelligence agencies.

The most visible organizations have been the FMLN, the Spanish acronym for Farabundo Marti Liberation Front, and its political arm, the Democratic Revolutionary Front, or FDR. Based in El Salvador, the FMLN is composed of guerrillas who receive Cuban aid and cooperate with both the Armed Resistance Unit and a Puerto Rican terrorist group called the FALN, according to the FBI.

In the most recent bombing, the United Freedom Front claimed responsibility for two bombs that damaged a Bronx Army Reserve Center Aug. 20. Kenneth Walton, deputy assistant FBI director in charge of the New York office, said at the time that the group is also known as the Armed Resistance Unit and the Revolutionary Fighting Group.

On Aug. 18, a small explosive device caused minor damage to the outside of a building housing a computer operation at the Washington Navy Yard. The FMLN, in a tape recorded call, claimed responsibility.

On May 12, explosions rocked military reserve centers in Nassau and Queens counties in New York. The FBI said the United Freedom Front was believed responsible.

Last Jan. 28, a man identifying himself as a member of the Revolutionary Fighting Group called United Press International to claim responsibility for a bomb that exploded near an FBI office in New York.

On New Year's Eve, four bombs exploded within 90 minutes of one another, rocking four government buildings, including New York City police headquarters and U.S. District Court in Brooklyn. A caller claimed at least two of the bombings were done by the FALN, while another caller said the PLO was responsible.