Slattery, whose decisive vote led to the downfall of a 1997 attempt to reintroduce the death penalty, said his decision was based on a last minute change of opinion.

He said he had followed the same path as the Speaker, moving from emotional support of the death penalty to opposition based on the possibility of a legal error leading to a fatal mistake.

"We cannot draft legislation which will ensure that you can be sure," he said.

Slavitt and Marini tried to minimize the potential dangers of procedural error. They rested their main argument on the grounds that in some cases, allowing certain prisoners to live amounts to an even greater injustice.

In the most horrible of murder cases, there is no suitable punishment other than death, Marini said.

He said he also supported the death penalty because he believed it has a deterrent effect and because the majority of Massachusetts residents have repeatedly professed their support for the measure.

Slavitt said death penalty opponents have a misconception that it is never right for governments to take other people's lives. But he said most people would agree that in some cases, such as in times of war, governments are justified in killing others.