Ahmed Maher, the leader of the 6 April Youth Movement, was arrested at Cairo Airport on Friday. He was returning to Egypt after a visit to the United States.

Khaled Al-Masry, media spokesman for the 6 April Youth Movement, confirmed the arrest of the revolutionary group’s co-founder, who was stopped at customs on his way into the country.

“We don’t know why he was arrested or what he is being charged with,” said Al-Masry, who said members of the group tried to reach Maher at the airport after police arrested him. Maher was not  made aware of the reason for his arrest before his phone was turned off, according to Al-Masry.

Maher had been in the US attending a conference in Washington, DC held by the New America Foundation. He participated in a panel on Egypt’s transition at the DC-based think-tank last week.