Cullen was arrested in December 2003 and had told investigators he might have killed as many as 40 people. He said he was able to move from hospital to hospital, despite suspicions he was killing patients, because the institutions did not report their fears to authorities.

Cullen's lawyer has said Cullen believed that his victims were terminally ill and that it was dehumanizing to prolong people's lives by artificial means.

'Going Through Depression'

Cullen mostly stared down at a desk in front of him during the 20-minute hearing, answering "Yes, your honor," or "No, your honor" to a series of questions from the judge. He looked pale and gaunt, his hands folded in front of him.

Cullen said he gave the patients an injection of a heart drug called digoxin, knowing it would kill them.

He gave no reason for his actions, and, as in previous court appearances, showed no emotion. But detective Scott Lessig, who helped investigate the deaths, said Cullen told authorities he acted out of depression.

"A lot of it had to do with his mental health situation," Lessig told reporters later. "He was going through depression, having a hard time."

A court official, speaking on behalf of the victims' families, said the families preferred not to comment on Cullen's plea.

Cooperated with Investigators

As part of an agreement with prosecutors, Cullen has cooperated with investigators in reviewing scores of suspicious deaths during his tenure at various hospitals to determine whether he had killed any other patients. He agreed to help authorities in exchange for a promise they would not seek the death penalty.

Cullen was arrested in December 2003 and had told investigators he might have killed as many as 40 people. He said he was able to move from hospital to hospital, despite suspicions he was killing patients, because the institutions did not report their fears to authorities.

The five new victims ranged in age from 49 to 81.