The National Transportation Safety Board has found no evidence that a tour bus that crashed last month killing 15 people in the Bronx was clipped by a tractor-trailer before it swerved and tipped over, federal regulators say.

In a preliminary government report issued Friday, investigators say the tour bus was traveling at its maximum 78 mph in the 45 seconds leading up to the crash. The posted speed limit on that stretch of Interstate 95, just south of the Westchester County line, is 55 miles an hour.

The report does not cite a cause of the crash. The NTSB said its analysis, "along with conclusions and a determination of probable cause," will be issued in its final report.

A final report will probably be issued next year.

The driver, Ophadell Williams, told investigators the bus was clipped by a tractor-trailer, which forced him to swerve and subsequently tip over.

The World Wide Travel bus ran off I-95 on its return to New York City from Connecticut on March 12. According to Friday's report, the bus veered to the right, crossed the shoulder, hit a barrier, and traveled 480 feet as it fell over before striking a stanchion that sheered off the vehicle's roof as the bus slid on its side.

There is no comment yet from Williams' attorney on the findings, and no charges have been filed.