German authorities say they have arrested three men at Frankfurt airport on suspicion they are members of the Somalian terrorist organization al-Shabab.

The Federal Prosecutor's Office said Monday the three German nationals were arrested Saturday after landing in Frankfurt from Kenya, on suspicion of membership in a foreign terrorist organization. They were identified only as Steven N., 26, Abdullah W., 28, and Abdulsalam W., 23, in line with German privacy laws.

Prosecutors said in a statement the three went to Somalia in 2012 and 2013 to join al-Shabab, and received weapons and combat training.

They say there is no evidence they had "concrete attack plans" upon their return but did accuse them of preparing an act of violence.

Prosecutors would not elaborate on the statement.