Federal inspectors looking into a deadly ceiling panel collapse in a Big Dig highway tunnel found that that ceiling had been designed with a smaller margin of safety than those in other tunnels in America, The Boston Globe reported Wednesday.

The Interstate 90 connector tunnel's drop ceiling panels were held in place by steel hangers suspended from bolts that were glued into the tunnel roof with epoxy. But there were no beams attaching the ceiling panels to the walls, and there were half as many bolts used as called for in the original design, according to a federal report obtained by the Globe.

"No redundancy was built into the ceiling in the event the hangers failed," the preliminary National Transportation Safety Board report said.

"The NTSB has researched other tunnels throughout the country and has found that significant redundancy is built into the ceiling design" so that the ceilings would not collapse when bolts fall out, the report said.

The report does not reach conclusions about the cause of the ceiling collapse because the investigation is not finished. The July 10 accident killed a motorist and led to a civil lawsuit and other federal and state criminal investigations into the $14.6 billion highway project.

The preliminary NTSB report said the joint venture company overseeing Big Dig construction, Bechtel/Parsons Brinckerhoff, decided to use epoxy bolts, rarely used for heavy objects, even after switching to heavier concrete ceiling panels to save money.

The NTSB report also suggests that flaws in construction by Modern Continental Construction Co. may have made the design more hazardous, the Globe said.

Federal investigators also have found no evidence the Massachusetts Turnpike Authority, the agency overseeing the Big Dig, rechecked the ceiling bolts after the I-90 connector opened in 2003, the report said. A similar finding was announced in October by the state's inspector general.

Modern Continental and Bechtel/Parsons Brinckerhoff have previously said they stand by their work on the Big Dig, and are cooperating with investigators.