Swedish police detained on Monday a second suspect linked to the Stockholm terrorist attack on April 7.

The man who drove a truck through the crowd in downtown Stockholm killing four civilians is a 39-year old Uzbek construction worker, Rakhmat Akilov. Akilov’s residence application was rejected in June 2016. He had been living in Sweden since 2014 and his motive for the attack remains unknown, DW reports.

No extremist organization has ever claimed responsibility for the attack.

The second suspect was arrested on Sunday following an investigation led by Sweden’s intelligence service, Säpo, Sverige Radio reports.

The public prosecutor must now convince the Court there is sufficient evidence to detain the suspect in custody. Police spokesman, Simon Bynert, confirmed the suspect is a man, but the Public Prosecutor will not release details of the suspect’s involvement.