The source said a preliminary investigation was opened after two complaints were filed against Mr. Ramadan, a well-known figure in the Middle East.
Islamic scholar Tariq Ramadan has had multiple rape allegations made against him since November, and was suspended by the world's top university as a result.
Tariq Ramadan is a world-renowned academic and teaches at Oxford UniversityWho is Tariq Ramadan?
A second unnamed woman on Friday also accused Ramadan of raping her in a hotel room in 2009. In a November statement, Oxford said it "acknowledged the gravity of the allegations" against Ramadan and emphasized "the importance of fairness and the principles of justice and due process".
Soon after, Ramadan was accused of making sexual advances on four women who studied under him as teenagers in Geneva.
"Many people are angry because I denounced someone whom people respect a lot", Ayari told The New York Times in October.
The United States barred him from the country for several years after the September 11, 2001 attacks, preventing him taking up an academic post there.
She described the alleged assault in her autobiography but at the time, did not name Ramadan to avoid legal issues.
He says all allegations are part of a "campaign of slander" by enemies.
The main complainant against Ramadan, the grandson of Muslim Brotherhood founder Hassan al-Banna, is former Salafist Henda Ayari, 40. In the wake of the Weinstein allegations, powerful men in many industries have been forced to reckon with their own misconduct, some of them facing criminal charges.