Four terror suspects arrested over the weekend in Sweden's second-largest city are being questioned and remain under suspicion of preparing a terror attack, prosecutors said Monday, September 12.

According to AP, authorities are being tightlipped about the arrests in the west coast city of Goteborg, which came on the eve of the 9/11 anniversary, and were followed by the evacuation of about 400 people from an arts center.

The Swedish Prosecution Authority said in a statement that investigation leader Agnetha Hilding Qvarnstrom must decide by Tuesday whether to release the suspects, or seek court approval to keep them in custody.

Prosecutors and police won't say whether the suspects are suspected of links to Islamist terror groups or other extremist networks. The prosecutors' statement said they were arrested without incident and noted that "the investigation is still in an early stage."