Officials say recent testing was unable to pinpoint the exact reason a 100-ton concrete beam fell from the new Biloxi Bay bridge.

The Federal Highway Administration said the beam was not fastened in place when it fell off the span last week. Investigators are not sure whether it was wind or vibration from nearby bridge work that caused the problem.

Transportation officials said construction vibration could have triggered the fall, but two men fishing in Biloxi Bay at the time said they did not recall seeing anyone working in the area.

Henry Humphrey, a retired industrial engineer, and his friend, Ron McCravey, were out fishing near the new span at the time of the accident. The two, who said they caught a number of fish that morning from their 13-foot boat, also questioned whether the wind was blowing hard enough to send the concrete beam tumbling off the bridge.

“First of all, it would be impossible for wind to blow that thing off the bridge, and if it was blowing that hard, there’s no way we could have been out there fishing,” Humphrey said.

The Mississippi Department of Transportation said Friday that GC Constructors, the company building the $338 million bridge, used a dive team to survey the bridge to make sure it had not been affected structurally. MDOT said it had not.

MDOT said the beam has not yet been pulled from the water.

The transportation department said the 781 beams already on the bridge are fine and the span will be safe to drive on when one side opens next month.