Norwegian police on Thursday set off a replica of the car bomb that ripped through Oslo's government district on July 22 in the first of two attacks that together killed 77 people.
Police spokesman Roar Hansen said investigators built the 2,100-pound (950-kilogram) device using fertilizer found at a farm belonging to Anders Behring Breivik, an-anti-Muslim extremist who has confessed to the Oslo bombing and a shooting massacre at a Labor Party island youth camp that killed 69 people.
The replica bomb was placed in a car at a military firing range, where it was detonated, Hansen said.
"The purpose was to measure the impact of such an explosion," he added.
Breivik is cooperating with police and helping them reconstruct how he carried out the attacks. But the 32-year-old Norwegian has pleaded not guilty to terrorism charges, claiming he is in a state of war to protect Europe from being taken over by Muslim immigrants.
A panel investigating authorities' response to the attacks on Thursday toured government offices in Oslo that were ravaged by the bombing.
Commission head Alexandra Bech Gjoerv told reporters she was deeply moved by the visit to the site, where eight people were killed by the explosion.
"It leaves a strong impression to see where people have been killed," Gjoerv said.
The block in central Oslo has been sealed off since the blast, with debris still being cleared from area. It remains unclear whether some of the most badly damaged buildings need to be torn down.
Breivik has confessed that he detonated the car bomb and later shot and killed 69 people gathered on Utoya island outside Oslo for the annual summer camp of the Labor Party's youth wing. Dozens more were injured.
Police have faced criticism for being slow to respond to the shooting at Utoya. A police helicopter was left unused, its crew on vacation, and a SWAT team had to switch boats on the way to the island because of an engine failure.