The FBI and Department of Homeland Security are looking into threatening phone calls received Monday and this morning at St. Joseph Mercy Hospital in Superior Township, the Washtenaw County Sheriff's Department said.

The male caller did not threaten the hospital, but mentioned Willow Run Airport and alluded to the Christmas Day terrorism attempt near Detroit Metropolitan Airport, the Sheriff's Department said. Willow Run Airport is east of Ypsilanti and 7 miles west of Detroit Metropolitan Airport.

The calls were not treated as bomb threats, "But the nature of the phone call was suspicious," Washtenaw County Sheriff's Det. Lt. Mike Trester said.

Trester said the calls were rambling and did not include any direct threats. The out-of-state caller alluded to the events of Christmas Day, when a 23-year old Nigerian man attempted to blow up a Northwest Airlines flight bound for Detroit Metropolitan Airport.

The FBI charged Umar Farouk Abdulmutallab Saturday with attempting to destroy a Northwest Airlines aircraft and placing a destructive device on the aircraft. He is being held at the federal prison in Milan following treatment for injuries at the University of Michigan Medical Center in Ann Arbor.

Health system spokesperson Lauren Stokes confirmed that Saint Joseph Mercy Health System has received some general security threats over the past several days, and that the security team has been monitoring the phone calls and working with law enforcement.

"After researching the threats and working with local law enforcement, we have no reason to believe these threats are credible," Stokes said in a written statement.

The hospital alerted the Washtenaw County Sheriff's Department about the first phone call after it received a message Monday on an answering machine. The FBI began an investigation after more calls were made this morning.

A local FBI agent declined comment, stating that was the agency's policy for active cases.

FBI spokesperson Sandra Berchtold said the FBI doesn't confirm or deny the existence of investigations, but "We are aware of the calls made to St. Joseph Mercy Hospital, and that's all I can say at this time."

The hospital has increased security and made security teams with dogs available on all shifts, Stokes said in a written statement.

Juliana Keeping covers higher education for AnnArbor.com. Reach her at julianakeeping@annarbor.com or 734-623-2528. Follow Juliana Keeping on Twitter