Alabama ? A car driver involved in the deadly crash of a school bus that nose-dived off an interstate overpass last week in Huntsville told investigators his steering system may have malfunctioned, a federal investigator said.

“He told us that he was experiencing a difficulty in steering his vehicle and as he continued with this difficulty, it got progressively worse until he felt he lost control of the vehicle,” said Gary Van Etten, who is leading the probe for the National Transportation Safety Board.

Witnesses to the Nov. 20 crash say that the car came up on a side lane next to the bus and apparently hit it.

The bus swerved into a concrete railing, then plunged 30 feet off the overpass onto a street below, killing four Lee High School students and injuring dozens of others among the 40 aboard.

Police said the 17-year-old driver of the car has since received death threats.