A federal judge in Boston said yesterday that there was mounting evidence innocent people were being executed. But he declined to rule the death penalty unconstitutional.

"In the past decade, substantial evidence has emerged to demonstrate that innocent individuals are sentenced to death, and undoubtedly executed, much more often than previously understood," the judge, Mark L. Wolf of Federal District Court in Boston, wrote in a decision allowing a capital case to proceed to trial.

He cited the exonerations of more than 100 people on death row based on DNA and other evidence.

"The day may come," the judge said, "when a court properly can and should declare the ultimate sanction to be unconstitutional in all cases. However, that day has not yet come."

Judge Wolf wrote that the crucial question for courts was "how large a fraction of the executed must be innocent to offend contemporary standards of decency."

His decision means that the case against Gary Lee Sampson, including the capital charges against him, will be tried next month. Mr. Sampson has acknowledged responsibility for three murders in Massachusetts and New Hampshire. Over a few days in 2001, he killed three men who had picked him up hitchhiking.

Mr. Sampson was willing to plead guilty to murder charges against him in Massachusetts and accept the maximum sentence available there, life in prison without parole. Instead, the federal government indicted him on capital charges based on the fact that the murders involved carjackings, a federal crime.

Judge Wolf, a former federal prosecutor and official in the Justice Department, was appointed to the bench by President Ronald Reagan. He appeared to be critical of recent changes in Justice Department practices in seeking the death penalty.

"Juries have recently been regularly disagreeing with the attorney general's contention that the death penalty is justified in the most egregious federal cases involving murder," he wrote.

In 16 of the last 17 federal capital prosecutions, Judge Wolf wrote, juries rejected the death penalty. A lawyer for Mr. Sampson, David A. Ruhnke, who specializes in capital cases, said Judge Wolf's numbers were outdated. The count, Mr. Ruhnke said, stands at 19 acquittals or life verdicts in the last 20 federal capital cases. The most recent acquittals were this month in Puerto Rico, which does not have the death penalty. Thirty-eight states do.

The Supreme Court has held that courts may take account of evolving standards of decency in deciding whether punishments violate the Eighth Amendment prohibition on cruel and unusual punishment. Those standards may be determined by looking at trends in, among other fields, legislation and jury verdicts.

"If juries continue to reject the death penalty in the most egregious federal cases," Judge Wolf wrote, "the courts will have significant objective evidence that the ultimate sanction is not compatible with contemporary standards of decency."

That statement suggests that the Justice Department, in seeking the death penalty more often and in more places, may actually be engaging in a counterproductive exercise from the perspective of supporters of capital punishment.

Judge Wolf acknowledged that there had been no legislative trend corresponding to the one reflected in the recent verdicts. "However," he wrote, "the increasing and disturbing new evidence concerning the execution of the innocent may generate legislation and jury verdicts which manifest a public consensus that the death penalty offends contemporary standards of decency and should no longer be deemed by the courts to be constitutionally acceptable."

He also noted that the department's policies about whether to take into account local opposition to the death penalty had changed. Until 2001, the policies said the absence of a local death penalty did not by itself justify a federal capital prosecution.

"It appears," Judge Wolf wrote, "that the fact that a state's laws do not authorize capital punishment may now alone be deemed sufficient to justify a federal death penalty prosecution."

A spokeswoman for the Justice Department, Monica Goodling, said it had an obligation to ensure the fair and consistent application of the federal death penalty.

One federal jury has sentenced a defendant to death in a jurisdiction that did not have its own death penalty since the federal death penalty was reinstated in 1988. The case was last year in Michigan.

The only other federal judge in Massachusetts to hear a federal death penalty prosecution in recent years later described what he had learned in The Boston Globe in 2001.

"The experience," Judge Michael A. Ponsor wrote, "left me with one unavoidable conclusion: that a legal regime relying on the death penalty will inevitably execute innocent people — not too often, one hopes, but undoubtedly sometimes."