ATLANTA (AP) _ Federal authorities said Friday they had received a few calls on a hotline offering clues about a group that claimed responsibility for a series of mail bombs, but the organization remained a mystery.

An FBI agent said a letter from the organization, sent to Atlanta television station WAGA and disclosed Thursday, contained information only people very close to the case could know.

The FBI in Atlanta asked anyone with information on the group, calling itself Americans for a Competent Federal Judicial System, to call a hotline established for tips in the case.

Agency spokesman Diader Rosario said Friday that some calls had been received since news of the group's letter broke Thursday.

The information received so far, however, was ''nothing exciting,'' Rosario said. He would not provide details.

The organization said it had ''assassinated'' 11th U.S. Circuit Court Judge Robert S. Vance and Savannah attorney Robert Robinson with mail bombs earlier this month.

Vance was killed at his home in Alabama Dec. 16 and Robinson in Savannah two days later. Two other bombs were intercepted - one mailed to the 11th Circuit in Atlanta and one to an NAACP office in Jacksonville, Fla. - before they exploded.

''Certainly it (the group) is something brand new for us, and another link in the chain,'' Rosario said.

''We believe the letter received by WAGA is authentic because it contained information which has not been made known to the public and which is known only to someone closely associated with the construction and mailing of the bombs and to law enforcement,'' said William L. Hinshaw, special agent in charge of the FBI investigation.

The Atlanta Journal-Constitution reported Friday that the letter contained an identifying code identical to one in follow-up letters sent to the bomb targets. The newspaper, which quoted unidentified sources, did not elaborate.

FBI officials in Atlanta would not comment further on the progress of the investigation.

The letter, which expressed anger at crimes by blacks against whites as well as court rulings involving school desegregation, threatened further violence against federal judges and civil rights officials.

Also Friday, investigators met with the central figure in a long-running school desegregation case that was mentioned in the threatening letter.

Roger Mills, lead plaintiff in the 21-year-old DeKalb County school desegregation lawsuit, said FBI agents held their second interview with him to discuss the letter's reference to the case. Agents warned Mills he might be a target of the group, Mills said, adding that he already was taking precautions.

''They showed me some of the letters that had been written, and they asked me for names of people that I know who would send letters like that,'' said Mills, who also is a member of the DeKalb NAACP's executive committee.

''We believe that this latest incident (the letter) is an effort to intimidate our association, to strike fear in our hearts,'' said Benjamin Hooks, executive director of the National Association for the Advancement of Colored People.

''We intend to go about our business, but we will most certainly be taking precautions,'' Hooks said from NAACP national headquarters in Baltimore.

The letter specifically referred to the case of Julie Love, a white Atlanta woman who was abducted in July 1988 and whose body was found last summer. Two black men were arrested; one has pleaded guilty to murder and other charges, and the other is awaiting trial.

''Protecting the innocent warrants a higher court priority ... than granting the blacks' demand for white teachers for their children,'' the letter said.

''The message Americans for a Competent Federal Judicial has for the judges is simple. If you want to live, you shall make protecting the civil rights of the innocent your highest obligation and you shall fulfill that obligation.''

The letter said Vance and Robinson were killed in reprisal for Ms. Love's death and that ''two more prominent members of the NAACP'' also would be killed. Neither Vance nor Robinson had any role in the case of Ms. Love.

The letter also said that any time a black man rapes a white woman in Alabama, Florida or Georgia - the states served by the 11th Circuit Court of Appeals - a federal judge, an attorney and an NAACP officer would be assassinated.

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