New Threats Made to 2 Bomb Targets May Aid Probe : Terrorism: The recipients are not named. Authorities admit that the letters could be a hoax.

ATLANTA--Federal officials, citing progress in their investigation of four mail bombings and attempted bombings in the South, said Thursday that “follow-up” threats mailed to two of the four targets may point the way to suspects in the crimes.

There is a strong possibility that the new letters, mailed this week, were sent by the same person or group responsible for sending the powerful package bombs that killed a federal judge Saturday and a lawyer two days later, the officials said.

“There have been follow-up letters sent since the receipt of the bomb parcels,” Leo F. Shatzel, special agent in charge of postal inspectors, told a news conference here. “It appears that these letters may have been sent by individuals who mailed the parcels.”

William L. Hinshaw, the agent in charge of the Atlanta FBI office, said: “There’s a great deal of interest in those letters.” He said there is “a very good possibility” that the letters were written to claim responsibility for the killings.

The federal investigators did not disclose what threats were contained in the letters or to which targets they were sent.

In addition to the bombs that killed Robert S. Vance, a judge of the U.S. 11th Circuit Court of Appeals, at his home in Mountain Brook, Ala., and Alderman Robert Robinson of Savannah, Ga., a bomb was defused at the 11th Circuit Court building here on Monday. A day later, another bomb was safely defused in the Jacksonville, Fla., office of the NAACP.

Officials said that their strongest suspects continue to be white supremacists and that civil rights organizations and the judiciary remain the most likely targets of possible new violence.

Rights groups have been increasingly vigilant since the bombings, and federal postal inspectors are screening packages addressed to the NAACP.

Although Vance was only the third federal judge to be assassinated in this century, civil rights officials said that they will hold a news conference today to outline previous attempts by hate groups to kill judges.

Investigators have recovered two bombs, which they say are similar to those that exploded--pipe explosives surrounded by nails. However, they have emphasized the threatening letters more than any evidence they may have gathered from their examination of the bombs.

The new letters contain language similar to a series of letters mailed last August to media organizations and various circuit courts around the nation, according to investigators.

The earlier letters cited what the writers contended were unfair court rulings and threatened to make massive poison gas attacks in retaliation.

At least one of these letters, headlined “Declaration of War,” was postmarked from Atlanta on the same day that a tear-gas bomb exploded at the Atlanta NAACP office.

Investigators are trying to find links between the August letters and those sent this week. Although they are enthusiastically pursuing this angle, they admit that the letters could be a hoax, sent by someone who learned about the crimes from news reports.

Therefore, the investigators continue to leave open any possibility, including the one that Colombian drug dealers may be behind the bombings, although that one has faded far into the background.

Hinshaw said: “We’ve got a few pieces of a puzzle, and we don’t know where they fit.”

In an attempt to enlist the public in the effort to solve the bombings, officials Thursday announced telephone “tip lines.” In Birmingham, the number is (205) 252-2416. The number in Atlanta is (404) 361-1092.

All information provided through these numbers will be kept confidential, the officials said.

The investigators, including agents from the FBI, the Bureau of Alcohol, Tobacco and Firearms, the U.S. Marshals Service, the postal service and local law enforcement agencies, are seeking common threads that connect the targets of the bombings to bomb threats.

So far, school desegregation suits brought by the NAACP seem the strongest connection: Robinson, an attorney, represented the NAACP in a suit in Savannah; the Jacksonville NAACP won a suit; and Judge Vance reinstated busing there, ruling last September.

Also, the state of Georgia has emerged as the chief geographical focus because all four bombs and an unspecified number of the letters were mailed from Georgia towns.

“I think it’s safe to say there are more investigations being conducted in Georgia than anywhere else,” Hinshaw said, because “we have more leads in Georgia. Just more things are going on.”