"We will not form an opinion until all the experts have testified," prosecutor Inga Bejer Engh said at an Oslo news conference Wednesday.
The Norway Post said court psychiatrists Synne Sorheim and Torgeir Husby, who completed the first psychiatric evaluation of Breivik, 33, were to testify Thursday. Several witness and experts have said the first psychiatric report, concluding Breivik is paranoid schizophrenic, is wrong, the Post said.
A second report concluded Breivik is sane.
Prosecutors are to hear from all four psychiatrists who completed the two reports.
Breivik has confessed to the July 22, 2011, killings of eight people in a bomb attack at a government building in downtown Oslo and a shooting spree that killed 69, mostly teenagers, on nearby Utoya Island. But he pleaded not guilty to murder and terrorism charges. He has claimed his actions were justified because he was waging war against multiculturalism and what he called a Muslim invasion of Europe.