Medicare paid billions of dollars to pharmacies with questionable billing patterns and possible criminal behavior, a new government report revealed Thursday.

The report, which examined Medicare claims from pharmacies in 2009, found that the program paid $5.6 billion to pharmacies with "suspicious" activities.

"What we are seeing in the data is extremely concerning," said Jodi Nudelman, a regional inspector general in New York who directed the research.

One Kansas pharmacy submitted more than 1,000 prescriptions for just two patients in a single year.

The report found that 2,600 pharmacies have questionable billing practices, which is only about 4 percent of the nation's pharmacies. Still, the losses add up since the average pharmacy bills Medicare about $1 million a year.

Researchers concluded that Medicare has limited safeguards in place, making it vulnerable to fraud, waste, and abuse.

"The findings call for a strong response to improve [program] oversight," researchers said in the report.

Medicare administrator Marilyn Tavenner said in a statement that the agency mostly agrees with the report's call to action. But she suggested that requiring private insurers to monitor and report suspicious activity may flood government officials with cases that turn out to be harmless.

The names of the 2,637 pharmacies identified in the report will be released to officials for follow-up.