Government agencies and major corporations account for many of the 15,000-plus complaints filed by returned vets since the end of September 2001, the Los Angeles Times reported Sunday.
The illegal actions have contributed to historically high joblessness among members of the Guard and Reserve, part-time service members returning from full-time duty in Afghanistan and other places.
A 1994 law requires employers to return members of the National Guard and Reserve to the same type of position they would have had if they had not been called to active duty.
The Labor Department and Office of Special Counsel say the number of complaints they have investigated rose from 848 in fiscal 2001 to 1,577 in the 12-month period ending in September 2011. The Defense Department handled another 2,844 cases in fiscal year 2011.
Records indicate the federal government was responsible for nearly a fifth of the formal complaints in fiscal 2012. The biggest alleged offenders were the departments of Defense and Veterans Affairs.
Among Fortune 500 companies, Walmart was the biggest alleged offender, with 29 complaints. UPS was close behind, with 19 claims between 2011 and 2012.