By Hope Yen, The Associated Press WASHINGTON — Federal authorities are stepping up investigations at Department of Veterans Affairs medical centers due to a sharp increase in opioid theft, missing prescriptions or unauthorized drug use by VA employees since 2009, according to government data obtained by The Associated Press. Doctors, nurses or pharmacy staff at federal hospitals — the vast majority within the VA system — siphoned away controlled substances for their own use or street sales, or drugs intended for patients simply disappeared. Aggravating the problem is that some VA hospitals have been lax in tracking drug supplies.