With the U.S. prison population fast approaching 2 million, critics of the justice system are complaining that there are "too many" people being locked up. But if what happened to police Sgt. Bruce Prothero is any indication, there may be too few.

In February, four men robbed a Maryland jewelry store where Prothero, a married father of five, was working part time as a security guard to supplement his police officer's salary. When Prothero chased the robbers out of the store, he was shot twice in the head and died an hour later at a local hospital. "He tried to give his family a better way of life," said Larry Sibley of the Baltimore County Fraternal Order of Police, "and it cost him his life."

The suspected triggerman, 29-year-old Richard Moore, had a rap sheet extending back 15 years. At age 16 he faced his first gun charge for armed robbery. Since then, he has been charged with at least five more weapons offenses, only one of which led to prosecution and a laughable 18-month sentence. Other charges on his criminal record include assault, battery, theft, resisting arrest, malicious destruction, and various drug offenses. One of Moore's accomplices, Donald White, was loose on the streets while awaiting trial for his second attempted-murder charge.

If the protection of society were the overriding goal of our criminal justice system, Moore and White would have been incarcerated - no complaints about America's "overcrowded" prisons, please - and Sgt. Prothero would be alive to protect his fellow citizens and raise his family.

A burgeoning prison population is nothing to be proud of. It is a sad commentary on the level of crime America still endures. Since 1990, the number of inmates held in either prisons or jails has increased dramatically, from 1.1 million to today's nearly 2 million. And given the pressures on the criminal-justice system, it makes sense to explore creative alternatives for non-violent offenders, including restitution and community service.

But the fact remains that most new state prisoners between 1990 and 1997 were violent offenders. Incarcerating these criminals saves lives and helps explain why violent crime rates dropped by more than 25 percent from 1991 to 1998. Indeed, in a rigorous study on the relationship between crime and incarceration, economist Steven Levitt of the National Bureau of Economic Research found that for each prisoner released as a result of overcrowding, nearly 15 additional crimes occur each year.

But rather than take overcrowding as a signal that more prisons need to be built, critics use it to justify liberal parole and probation policies and to lobby for lenient prison sentences. As a result, violent criminals still too often serve only a fraction of their sentences or receive sentences that do not match the severity of their crimes.

Take the case of three juvenile offenders in Florida - 16-year-old Sylathum Streeter, 17-year-old Victor Lestor and 16-year-old Curtis Shuler - who went on a murderous shooting spree in 1998. This band of merry gangsters attempted three carjackings and shot four people, two of them fatally. They later broke into an innocent family's motel room and shot four more people, including a 10-year-old girl. (Fortunately, all four survived.)

According to the Florida Department of Juvenile Justice, both Shuler and Streeter were on probation at the time. Shuler had a record of aggravated assault charges, burglary and petty larceny, while Streeter had a juvenile record of assault and battery and armed robbery. Lester had a previous conviction for larceny. The question is, why were they on the street?

None of the innocent people targeted by these thugs would have been harmed if violent criminals were kept in jail where they belong. Nor would Sgt. Prothero. As his wife put it, "My husband's job was to help keep people who do these heinous crimes off the streets. The fact that they're out there in public, not behind bars -- that's unacceptable."

Yes, it is. We should not celebrate the fact that America's prisons are so full, but on balance it's not such a bad thing -- provided they're full of criminals.

Robert E. Moffit Ph.D.  is the director of domestic policy studies and David B. Muhlhausen is a senior policy analyst at The Heritage Foundation, a Washington-based public policy research institute.