BIRMINGHAM, Ala. (AP) _ Investigators have disproved a dying man's claim that he planted the bomb that killed four black girls at a church in 1963, two prosecutors said Friday.

The investigations, said U.S. Attorney Frank Donaldson, reveal that Gary A. Tucker ''is the confessor of a crime he did not commit.''

Tucker, a 54-year-old man who is dying of cancer at a Veterans Administration hospital in Tuscaloosa, told officers last week that he and another person placed the bomb at the 16th Street Baptist Church. The bomb exploded on Sunday, Sept. 15, 1963, killing the four girls who were in a lounge preparing to take part in a youth program.

Donaldson said that Tucker ''made several statements. All of them have been thoroughly investigated. There is no question about it. Tucker's statements are not credible. Efforts were made to corroborate his statements. This could not be done. In fact, they were disproved.''

Donaldson said he was disappointed because he had ''hoped this horrible crime would be finally laid to rest.''

Jefferson County District Attorney David Barber said that a person Tucker named as an accomplice was in a state prison at the time of the bombing.

''When compared with the knowledge gathered in previous investigations, and the facts we learned in this investigation, his story is simply not credible,'' Barber said at a news conference with Donaldson.

Relatives and friends of Tucker had said he suffers from hallucinations and that they believed he was merely seeking attention.

A Ku Klux Klansman, Robert Chambliss, was convicted in 1977 of murder in one of the bombing deaths and died in prison eight years later while serving a life term.

Donaldson and Barber said their files on the case are still open, since there is no statute of limitations on murder, but they said they have no leads.

Barber said that several things in Tucker's statement were inconsistent - ''where the church is, descriptions of three different automobiles he said he drove to the church, where he lived, where he worked.

''Nothing he said was consistent with the facts other than things he gleaned from newspapers.''

Tucker, Barber said, ''said he realized he is dying, that he doesn't have long to live and that he wanted to get this off his conscience.''

Donaldson said he hoped that news accounts of the investigations into Tucker's statements will cause ''some people to come forth with information that is helpful. We might get a deathbed statement sometime. I had hoped that might have been the case this time.

''The crime was so horrendous, we have to follow up on anything, especially so-called confessions.''

Barber said he sympathized with ''the families of the victims of this atrocity for having to relive their nightmare for the past week.''