OSLO — A lawyer representing the anti-Muslim extremist who carried out the massacre in Norway sought to fill in some of the wide gaps in public knowledge about his client on Tuesday, portraying him as a “very cold” person who lived in his own world, buttressed by drugs and the belief that he was a warrior doomed to die for a cause others did not comprehend.

The lawyer, Geir Lippestad, declined to say whether his client, Anders Behring Breivik, 32, would plead insanity as a defense when his case comes to trial. However, he told a news conference, “this whole case has indicated that he is insane.”

Police and court officials have said Mr. Breivik has admitted to detonating a large bomb in Oslo that killed eight people and then killing 68 mainly young people at a summer camp run by the ruling Labor Party on the nearby island of Utoya. The attacks on Friday amounted to one of the worst massacres in postwar Europe.