Radicals who bombed a factory here Saturday to protest a visit by President Carter this Wednesday to a nearby mill town are threatening more violence, the FBI said yesterday.

The radical group, which has claimed responsibility for several bombings, left a letter referring to Carter's scheduled participation in a town meeting Wednesday in Clinton, 10 miles from here.

While the President "wines and dines," the letter said, the bombers remember "our people," whom they said were "brutalized in prison."

Richard Bates, in charge of the Boston FBI office, said that in a telephone call after the bombing, a man identifying himself as a member of a radical group, said "there would be more," but did not elaborate.

The group, the Sam Melville-Jonathan Jackson Unit, claimed responsibility for the explosion that blew out windows and damaged the foyer and office of the Ideal Roller and Graphics Co.

The letter also demanded the release of a Puerto Rico nationalist convicted of killing a guard while trying to assassinate President Truman in 1950 and four other Puerto Ricans convicted of wounding five congressmen in the House in 1954, the FBI said.

The Secret Service said yesterday that security plans for Carter's visit to Clinton had not changed.

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Meanwhile, in Tower City, Pa., a mine safety official said there is hope that a "miracle" saved five men in a disaster March 1 at the Kocher Coal Co mine.

Teams are working around the clock at the mine, where four men were killed following flooding of a tunnel. One miner was rescued march 1.