Audio of the 911 calls.

Federal authorities say George Zinkhan has a ticket for a flight to the Netherlands for May 2 and they can’t find his passport.

Zinkhan has a house in Amsterdam, where he teaches part-time. It was unclear when the ticket was purchased.

In a federal court affidavit Monday, according to the Associated Press, an FBI agent says no one knows where Zinkhan is. Officials have searched his office and he hasn’t contacted family, friends or students.

Authorities say they found Zinkhan’s passport wallet, but the passport is missing.

FBI Special Agent Gregory McClendon said Delta Air Lines confirmed Zinkhan has a ticket to Amsterdam for May 2.

“He may change the date and attempt to leave early,” McClendon said in the affidavit, which seeks a federal arrest warrant.

The affidavit did not say when the ticket was purchased, although a family friend told investigators that Zinkhan’s wife had said Zinkhan scheduled a trip to Amsterdam. A colleague said Zinkhan had recently purchased a phone that could be used internationally.

Also Monday, Zinkhan’s brother Chris Zinkhan said in an e-mail to the AP he and other family members were in contact with Athens police and the FBI.

“Our thoughts and prayers are with the victims, their families, and their friends,” Zinkhan said.