Concern about overcrowded jails and prisons, combined with a laudable desire to give people who have made mistakes second chances in life, have prompted many Americans to rethink sentences for criminal offenses. That is occurring at the local, state and federal levels.

But mistakes already have been made. In some jurisdictions, menaces to society have been released from custody, only to seek out new victims.

One error should never, ever be made, as the British can explain to us. It involves men and women who have been involved in terrorism-related crimes.

British officials said Monday they are taking emergency action to stop releases from prison of people who have served only half their sentences for terrorism crimes. Yes, half. It seems the British for some reason decided that even violent extremists were entitled to consideration. So, some convicted of terrorist acts were being set free long before their scheduled releases.

The price of that failure in judgment has been paid in blood. Monday’s announcement came after two murderous attacks in London by men convicted of terrorist violence who were released early. One, in November, resulted in the deaths of two innocent victims. The other attacker, this week, stabbed two people before police shot him.

Releases of terrorist convicts who have served just half their sentences will end, according to British Justice Secretary Robert Buckland. In the future, convicted terrorists will serve at least two-thirds of their sentences — and cannot be released before serving the full terms unless the Parole Board approves.

Sentencing reform in the United States needs to place those convicted of terrorism-related offenses in a special category. Early releases should be considered rarely, and then only when there is no doubt whatsoever that rehabilitation has been accomplished.