U.S. prisons now house a record 2.1 million inmates, but a U.S. Census report says that prison growth began to level off during the early 2000s, the Washington Post reported Sept. 27.

The 2006 census data in the American Community Survey showed that the prison population grew 4 percent since 2000, a far more modest increase that the 77-percent growth between 1990 and 2000. Experts generally agree that tough drug sentencing laws led to the spike in prison populations during the 1990s.

"The growth wasn't really about increasing crime but how we chose to respond to crime," said Allen J. Beck, deputy director of the Bureau of Justice Statistics. "When you increase the likelihood of a person going to prison for a conviction, and then you increase how long you keep them there, it has a profound effect."

Most of those laws remain in effect, leading experts to speculate that the slowing growth of prison population may be due to a lower crime rate in the 1990s, the fact that many potential offenders are already in prison, and that many of those sentenced in the heyday of the drug war are now being released from prison after serving their time.

"All those people who were in prison are starting to come out ... so the number that is going in is approaching the number going out," said Christy Visher of the Urban Institute's Justice Policy Center.

The crime rate is starting to rise again, however, and many released prisoners on probation are likely to end up back in prison again within three years, meaning the prison population could begin to rise faster once more, said Visher.