FBI agents investigating the bombing that killed a custodian at National Airport Saturday are concentrating on a theory that the victim unintentionally stumbled into a plot against somebody else.

"It would appear that the bomb was aimed more at an individual or group of individuals," said Robert G. Kunkel, special agent in charge of the FBI Alexandria Field Office, which is handling the investigation.

A bomb designed to create vast damage at an airport would probably have contained a different kind of explosive ad have been triggered by a timing device, Kunkel said.

The bomb that killed Julious B. Rogers, 51, was designed to go off when the lid was lifted on a toolbox. The tool box was packed in a cardboard carton, and the carton was sealed with "distinctive black tape," Kunkel said.

"It's likely Rogers found the box somewhere, took it to the locker room, and started to examine it while he was on his break," Kunkel said.

The explosion, about 11 a.m. Saturday, killed Rogers, but did little damage to the employee's locker room in the airport. There was little effect on airport operations.

In the early hours of its investigation, the FBI concentrated on learning what it could about Rogers. "We don't think he's the kind of individual who would make the bomb, or the kind of individual who would have been the recipient of a bomb," Kunkel said.

Rogers, a widower, lived alone in an apartment in Southeast Washington. Coworkers and supervisors described him to reporters and investigators as as pleasant and cooperative. He was also something of a pack rat, according to sources, which may explain why a shipping carton caught his attention in the first place.

The origin of the shipping carton containing the toolbox is receiving the most intensive FBI attention, Kunkel said.

Rogers is survived by four daughters and a son.