Shell companies posing as legitimate medical billing companies have been bilking Medicare of hundreds of millions of dollars by submitting bogus claims, a Reuters probe has found. Some of the shells submit massive sham bills; others pay doctors or patients kickbacks for their claims or Medicare ID numbers. They fraudulently bill and get paid millions before the government finds out what's happening. The Centers for Medicare and Medicaid want to fight the fraud but say they don't have enough resources to review companies' backgrounds before they pay, though the health care law passed last year helps somewhat. Reuters estimates that $1 billion of the almost $3 billion in fraudulent claims made since 2007 was billed by shell companies. One nationwide fraud ring used 118 shell companies to bill over $100 million before its members were indicted.