At least two people were killed and 10 injured in a shooting at a nightclub in Oslo, Norway, early Saturday morning, according to Norwegian authorities, prompting officials to raise the terror alert.

Investigators said the suspect, identified as a 42-year-old Norwegian citizen originally from Iran, was arrested after opening fire at three locations in downtown Oslo.

Police said two men, one in his 50s and and the other his 60s, died in the shootings.

He said the agency, known by its Norwegian acronym PST, first became aware of the suspect in 2015 and later grew concerned he had become radicalized and was part of an unspecified Islamist network.

Police said civilians assisted them in detaining the man in custody, who was being held on suspicion of murder, attempted murder and terrorism, based on the number of people targeted at multiple locations.

Olav Roenneberg, a journalist from Norwegian public broadcaster NRK, said he witnessed the shooting.

"But then I heard that it was a shooting and that there was someone shooting with a submachine gun," Nybakken told Norwegian broadcaster TV2.

Norwegian Prime Minister Jonas Gahr Støre called the shooting a "cruel and deeply shocking attack on innocent people.".

He said that while the motive was unclear, the shooting had caused fear and grief in the LGBTQ community.

Christian Bredeli, who was at the London Pub, told Norwegian newspaper VG that he hid on the fourth floor with a group of about 10 people until he was told it was safe to come out.

Police said the suspect had a criminal record that included a narcotics offense and a weapons offense for carrying a knife.

Inge Alexander Gjestvang, leader of FRI, a Norwegian organization for sexual and gender diversity, said the shooting shook the Nordic country's LGBTQ community.

The summit's host, German Chancellor Olaf Scholz, tweeted, "The Norwegian people can be sure of our sympathy.

Last year, a Norwegian man armed with knives and a bow and arrow killed five people in a town in southern Norway