“There are no grounds for keeping him in custody,” Gothenburg prosecutor Lars Lindman said in a statement.

The man had sat in custody since the early hours of Monday morning when he was arrested at his student halls of residence in Gothenburg after posting a threatening message on the US-based internet forum 4chan.org.

The 33-year-old Chalmers University of Technology student on Tuesday expressed “deep regret” over his actions and has explained to police that the whole incident was a very bad joke.

Swedish police sought the help of the FBI to trace the 33-year-old from his threatening post on the popular website. In his post he had explained that his girlfriend had recently broken up with him and that he wanted to die.

“On Monday I plan to take my gun … to the school and shoot as many people as I can before the police come and shoot me,” he wrote.

The poster also referred to recent school massacres in Finland.

“You’ll see. I will beat that Finnish bastard’s record, since Swedish police do not exactly seem to be the fastest at getting to the scene.”

Sweden was hit by a wave of copycat threats on Monday and Tuesday in what experts described as a typical pattern for this type of offence.

A school in Västervik was closed as a precautionary measure after threats of a shooting were forwarded by mail on Monday. The decision was taken to keep the school closed on Tuesday as police had been unable to determine the threat’s source.

In northern Sweden a 14-year-old boy took dynamite and a detonator to a school in Ramsele, and a 16-year-old in Piteå admitted to threatening to take a gun to school on Tuesday. Both were released after preliminary police interviews.

Later on Monday evening, a boy threatened to use a rifle to kill pupils and staff at a school in Borås in western Sweden. The school remained open under police surveillance on Tuesday.

“This type of crime has a contagion effect which is evidently greater than with other offences,” said psychiatrist Ulf Åsgård to the Aftonbladet newspaper.