Community Health Systems Inc said Friday it received a subpoena from the U.S. government in connection with an investigation of possible improper claims submitted to Medicare and Medicaid, sending its shares down 9% after-hours.

The subpoena, from the U.S. Department of Health and Human Services and dated March 31, requests documents from all of the company's hospitals and appears to concern emergency department processes and procedures, the company said in a securities filing.

It also requests information about Community's relationship with emergency department physicians, including financial arrangements.

Community said it does not know if it is related to allegations made in a lawsuit against it by Tenet Healthcare Corp. Tenet, a rival hospital operator, filed a lawsuit against Community earlier this week accusing it of admitting patients for needless stays and bilking the U.S. government and private insurers.

The company said it is cooperating fully with the subpoena and is currently unable to forecast the outcome of the probe.

Community Health shares fell to $29 in after hours trading from their close on Friday at $31.90 on the New York Stock Exchange