OSLO (Reuters) - An Oslo court ruled on Monday that Norwegian police could continue to detain a Russian teenager while they investigate suspicions that he was plotting an attack with a primitive explosive device.

Armed police guard the courtoom in Oslo District Court, where a judge is deciding on the detention of a 17-year-old Russian, suspected of planting a bomb-like device on Saturday, in central Oslo, Norway April 10, 2017. REUTERS/Gwladys Fouche

The 17-year-old was detained on Saturday night, accused of possessing a container of lighter gas with nails taped around it in what his lawyer said was a “boyish prank”. The bomb squad used a robot to blow up the object in central Oslo. No one was hurt.

Police across the Nordic region went on heightened alert after a hijacked truck plowed into pedestrians on a Stockholm shopping street on Friday, killing four, in what Swedish police said appeared to be an Islamist attack.

In Oslo, judge Mads Wilhelm Ruland ruled police could detain the Russian for two weeks, the maximum allowed for a minor, while they investigate under anti-terrorism laws.

“He had a container of lighter gas taped with nails around it,” police prosecutor Kathrine Tonstad told reporters.

Police said the motive of the youth, whose name has not been released, was not known and it was unclear whether he had acted alone.

Wearing a yellow jumper with grey stripes, the teenager appeared impassive, sometimes leaning towards his lawyer to talk. Most of the hearing was conducted behind closed doors at his and the authorities’ request.

Outside the courtroom, armed police searched attendees’ belongings in a rare display of high security in the Nordic country.

After the Stockholm killings, Norway’s PST security police raised the risk of an attack in Norway to “probable” from “possible”.

The youth’s lawyer said he was not an extremist.

“He disagrees with Islamists and is against violence. He had not planned to hurt anybody,” Aase Karine Sigmond told Reuters. “This was a boyish prank.”

The teenager would consider whether to appeal against his continued detention, said the lawyer.

The youth, from Russia’s southern Caucasus region, came to Norway as a 10-year-old with his family and had applied for asylum.

The PST said it had been in “preventive” contact with him earlier after concerns were raised that he may have been radicalised.

But a man who used to train at the same wrestling club as the youth told Reuters: “He was a normal boy... There was nothing extreme about him.”

He was detained after a witness alerted police to a person on his knees holding a suspicious package beside a car in a dark street.

“If it was a bomb he had made, I would be surprised if he was completely alone,” Thomas Hegghammer, a researcher at the Norwegian Defense Research Establishment who focuses on violent Islamism, told Reuters. “It is not easy to build a bomb.”