Bomb Carries Maker’s ‘Signature,’ Experts Say : Terrorism: Forensic scientists can tell roughly where a bomb was made, and often who it was that made it.

Like footprints in snow, the four mail bombs that have killed two people in the South since Saturday left evidence of those who made them, forensic scientists said Tuesday.

“Certain people make certain types of bombs,” said Robert Barry, a retired FBI agent and head of the University of Southern California’s Center for Administration of Justice. “If they’ve been successful with one type of bomb, they tend to stick with it and make others like it.”

In the fourth bombing incident in a series that began Saturday, police Tuesday disarmed a parcel bomb that had been mailed to the NAACP office in Jacksonville, Fla.

Meanwhile, police and FBI officials confirmed the bomb that killed Savannah, Ga., Alderman Robert Robinson Monday was similar to one that killed U.S. 11th Circuit Court of Appeals Judge Robert S. Vance Saturday and another that forced Monday’s evacuation of the Atlanta courthouse where the 11th Circuit has its headquarters.

Law enforcement officials familiar with the investigation have not revealed the type of explosive or many details about the bombs.

Barry, who was the FBI’s senior resident agent at Los Angeles International Airport for 17 years, and other forensic experts said that identifying the explosive in the two bombs that exploded would be the investigators’ first task.

Dynamite and other American-made explosives leave residues and trace materials when they explode that differ from foreign-made plastic explosives such as Semtex. It will, of course, be easier to identify the agent used in the bombs that were disarmed.

“That bit of information is of enormous value,” said David Fine, vice president of Thermedics Inc., a Massachusetts-based firm that manufactures explosive detection devices for law enforcement agencies. “That is going to give (investigators) an idea of what and who to look for.”

For example, forensics experts said, domestically produced explosives would lend credence to theories that the bombings were racially motivated. Plastic explosives might hint that foreign drug lords were involved.

“They are going to know whether the bomb was domestically made or foreign made, where the battery that detonates the thing was made and on what date,” Fine said. “They’re going to know what kind of switch was used, whether the person who built it knew what he was doing and how skilled he might be. In short, it will tell them what type of person they’re dealing with--if not the person exactly.”

Investigators in Savannah turned the modest town house where Robinson maintained his one-man law practice into a crime laboratory. The walls and ceilings of the upstairs office where the bomb exploded were deeply scarred by the nails that the bomb contained and parts of the carpet were thick with blood.

Detectives divided the floor into rectangular grids and picked each portion painstakingly clean and vacuumed it for evidence. Downstairs, agents from the federal Bureau of Alcohol, Tobacco and Firearms worked elbow-to-elbow to sift through the debris.

Asked whether his agents had found information that might point to the origin of the bomb, bureau agent Thomas W. Stokes said: “They’ve found plenty.” He declined to elaborate.

Paul Snabel, special agent in charge of explosive technology at the bureau in Washington, said that investigators would collect as much physical evidence as they could at the bomb sites. Then, he said, “it’s just hard-core police work and luck.”

Snabel said that mail bombs are rare in the United States. Since 1979, he said, there have been 139 instances of mail bombings, with 11 people killed, 89 people injured and $90,000 in property damage.

Snabel said that the construction of mail bombs requires “a certain level of expertise” because it is difficult “to make one that goes off exactly where and when you want it to.”

Douglas Jehl contributed to this story from Savannah.