The US government reportedly has upgraded its investigation into reports that tailgates are falling off the back of some General Motors pickup trucks without warning.

According to the Associated Press (AP), the National Highway Traffic Safety Administration on Monday said it was reviewing tailgate support cables installed in 1998-2004 model years of the GMC Sonoma and Chevrolet S-10 pickups and the 1998, 1999 and part of the 2000 model years of the Chevrolet Silverado and GMC Sierra.

GM officials reportedly said the upgraded investigation covered 3.1 million vehicles.

AP noted the move advances an NHTSA investigation, which was opened in February 2004, of the Sierra from model years 1999-2002; the Silverado from model years 1999-2003; the 2002 Cadillac truck and the 2002 Chevrolet Avalanche.

NHTSA told AP consumers have reported about 50 injuries due to the alleged defect, which involves the breaking of the support cables because of metal fatigue, leading to the separation of the tailgate.

GM spokesman Alan Adler told the news agency the company was cooperating with investigators. He noted that for the smaller pickups, the Sonoma and the S10, “those trucks are used for very different purposes and we have had comparatively fewer numbers of warranty claims or complaints.”

The Associated Press noted that GM announced a recall of four million 2000-2004 pickups in March 2004 because of breaking tailgates – the recall involved 2000-04 Chevrolet Silverados and GMC Sierras built between October 1999 and October 2003. Some 2002-04 Chevrolet Avalanche and Cadillac Escalade EXT trucks built between March 2001 and October 2003 were also recalled.

Adler told the news agency replacement parts for the vehicles became available in September 2004 and about 40% of the recalled vehicles have been fixed – GM has sent reminders to vehicle owners who have not yet had their trucks repaired every two months.