Norwegian police are investigating the possibility there might have been a second gunman involved in Friday's deadly assault on a summer youth camp.

Police already are questioning one suspected gunman, a native Norwegian, but the country's national news agency NTB said Saturday that witnesses on Utoeya island where the attack occurred have told police two people were involved. The man already in custody was disguised as a policeman, wearing a sweater with a police emblem on it, but the witnesses said the second man was not.

Norway reeled with horror at Friday's twin attacks that left at least 91 dead — seven in a massive bomb blast attack on the government headquarters in Oslo and at least 84 in the subsequent gun attacks on the youth camp, where several hundred teenagers had gathered as part of a summer program sponsored by the country's ruling Labor Party.

Prime Minister Jens Stoltenberg called the assaults – the worst in Norway since World War Two – “a national tragedy. It is a nightmare.” He called the bombing and shootings “bloody and cowardly attacks” and said the island has been turned from “a paradise into hell.”

Police are searching the lake surrounding the island about 30 kilometers north of Oslo for more bodies. Police say the suspect, identified by news agencies as 32-year-old Anders Behring Breivik, also is suspected of carrying out the bombing that targeted the government headquarters.

Police say that Breivik was cooperating in their investigation. They described him as a “fundamentalist Christian” with political views that leaned “to the right.” Police say he had posted anti-Muslim rhetoric online and news accounts said that he has been a strong opponent of multi-culturalism.

Norwegian media also reported that Breivik said on his Facebook page that he liked to play computer games, such as World of Warcraft and Modern Warfare Two.

Breivik operated a company called GeoFarm that was involved in growing vegetables, melons, roots and tubers. A farm cooperative said Saturday that it sold Breivik six tons of fertilizer in May, an amount it described as a “relatively standard order” for a farm like Breivik's.

Fertilizer can also be used to make bombs. Norwegian media say the massive bomb that exploded at the government building was made of fertilizer.

Mr. Stoltenberg said the “brutal” attack on “innocent youths” would not take away Norwegians' feeling of safety. He said safety was a pillar of society that Norwegians had taken for granted, and he stressed that the main focus is on saving the lives of those hurt in the attacks.

Authorities have not determined a motive for the attacks.

Eskil Pedersen, a leader of the Labor Party youth wing and a survivor of the attack, said the group “will not let the terrorist win.” He said the group will continue to work hard for the party in honor of those who were killed.

The building that was bombed in Oslo houses the office of the prime minister. He was not there at the time and was not harmed.

U.S. President Barack Obama condemned the attacks and expressed his condolences for the loss of life.

U.S. Secretary of State Hillary Clinton added that the United States is ready to support Norway as it seeks to bring those responsible to justice.

U.N. Secretary-General Ban Ki-moon, through his spokesman, said he was shocked by the attacks and said the United Nations stands with the people of Norway “at this terrible moment.”