NORTHFIELD, Vt. (AP) —
    Federal investigators are back at work probing the derailment of a Washington, D.C.-bound Amtrak train that derailed after hitting rocks in Vermont, sending two cars down an embankment and injuring seven people.
    One of the victims, a crew member, was seriously hurt and airlifted to a hospital.
    The Vermonter, carrying 98 passengers and four crew members, had been on the tracks for about 90 minutes when it hit rocks that had fallen from a ledge onto the tracks in Northfield, about 20 miles southwest of Montpelier at around 10:30 a.m. Monday.
    The locomotive and one of the passenger coaches tumbled down an embankment. Three passenger cars left the tracks but remained upright.
    "This was a freak of nature," Gov. Peter Shumlin said.
    One of the injured people was airlifted to a New Hampshire hospital. Six others went to a local hospital with injuries including neck, back and shoulder pains and lightheadedness.
    Amtrak said a crew member was hospitalized with non-life-threatening injuries but four other people were released by Monday evening.
    Investigators from the Federal Railroad Administration and National Transportation Safety Board arrived on the scene Monday afternoon.
    Passenger Bob Redmond, of Bay City, Michigan, was taking a foliage tour when the train derailed. He looked outside the window and saw the car that had been ahead of his was now alongside him.
    "It was just going the other way, and we started tipping sideways and down we went," he said.
    Federal records show two companies that have operated that stretch of track have had four accidents since 2006 that could have involved debris on the tracks.
    Federal safety rules for tracks that carry passengers require at least two inspections every week, with at least one day between inspections. State officials said a freight train had passed over the tracks Sunday night with no problems.
    Numerous derailments around the world have been caused by debris on tracks, many linked to heavy rains that trigger slides. In 2010, a train in Beijing hit mounds of debris left on the track following a landslide, killing 19 people.
    The region near Monday's derailment received 2.5 inches of rain between Thursday and Friday, the National Weather Service said.
    Amtrak planned to bus passengers booked on the Vermonter to and from Springfield.