Dutch prosecutors are investigating a possible “terrorist motive” behind the shooting on a tram in the city of Utrecht in which three people were killed and five wounded.

A Turkish-born man, 37-year-old Gokmen Tanis, was arrested after a seven-hour manhunt on Monday by security forces and remained in custody.

Two other men were arrested on Monday and released after being cleared of involvement. A fourth suspect was arrested on Tuesday. “His role is being further investigated,” prosecutors said in a statement.

They described Tanis as the chief suspect.

“Up to this point, a terrorist motive is seriously being considered,” prosecutors said in a statement, citing “the nature of the shooting and a letter found in the getaway car.”

But it remained unclear whether the shooter acted on political beliefs or some personal vendetta. “Other motives are not being ruled out,” the statement said.

Tanis has a history of run-ins with law enforcement. He was convicted for illegal weapons possession in 2014 and for shop lifting and burglary earlier this month.

He was released from custody on March 1, after having been detained on suspicion of rape, the Utrecht District Court said in a statement released after his arrest on Monday.

He is due to appear in court in July in relation to the rape charge, it said.

Under Dutch law Tanis must be brought before a judge by Thursday but does not yet have to be charged.

The three Dutch victims were identified as a 19-year-old woman and two men aged 28 and 49. Three others, ranging from 20-to-74 years old, were critically injured in the shooting.

Prosecutors said they had so far not been able to establish a connection between the victims and the suspected gunman.

Dutch Prime Minister Mark Rutte convened crisis talks immediately after the incident, which came three days after a lone gunman killed 50 people in mass shootings at two mosques in the city of Christchurch, New Zealand.