Federal prison officials aren't reading all mail to and from convicted terrorists and other high-risk inmates, a security gap that could prove deadly, a Justice Department review concluded Tuesday.

Moreover, prison investigators read less inmate mail now than a year ago at seven of 10 prisons surveyed by the Justice Department's inspector general.

"The threat remains that terrorist and other high-risk inmates can use mail and verbal communications to conduct terrorist or criminal activities while incarcerated," concluded the inspector general's report of U.S. Bureau of Prisons facilities.

The mail investigation was spurred, in part, after three convicted terrorists at a federal maximum-security prison in Florence, Colo., were found to have written an estimated 90 letters between 2002 and 2004 to Islamic extremists — including some with links to the March 11, 2004, attacks on commuter trains in Madrid. Some of the letters later surfaced in the hands of a terror suspect who used them to recruit suicide operatives.

The Madrid attacks killed 191 people and wounded more than 1,700.

Following that discovery, the Bureau of Prisons took steps to limit high-risk inmates' mail and telephone calls, the inspector general found. It also hired more Arabic translators and sought to better analyze mail for clues to suspicious or criminal activity.

But limited funding, in the face of a growing inmate population, has hindered those efforts, the inspector general's report concluded.

More than 191,000 inmates were housed in federal prisons and detention facilities as of July, a 70 percent increase over the last decade. An estimated 10 percent of those inmates, or 19,720, are considered high risk — including domestic and international terrorists and gang leaders and members. The number of high-risk inmates similarly increased, by 60 percent, during that period.

By contrast, federal prisons' staff grew by 14 percent over the decade — from an estimated 30,200 to 34,600.

The review examined mail monitoring systems at 10 federal prisons and detention centers: in Brooklyn; Manhattan; Sheridan, Ore.; two each in Florence, Colo., and Beaumont, Texas, and three in Allenwood, Pa.

The maximum-security prison in Florence, Colo., for example, houses about 400 of the most dangerous and violent inmates in the federal system. They include the 1993 World Trade Center bombers, convicted Unabomber Ted Kaczynski, convicted spy Robert Hanssen, and leaders of violent street gangs.

The Bureau of Prisons expects its staff to read 100 percent of high-risk inmate mail, including translating documents written in foreign languages. But the agency does not require its staff to keep track of how often the staff does so, the review found, noting that inspectors were told in each of the 10 facilities that the 100 percent target "was not being consistently met."

Monitoring other inmates' mail proved spotty, too, the inspectors found.

The two mailroom staffers at the high-security Colorado prison were only able to read about 1.8 percent of the 2,464 pieces of incoming mail over a five-day period in November 2005, the inspector general's review found.

The Metropolitan Correctional Center in New York had the best rate, with its two mailroom staff randomly reading half of the 6,100 pieces of mail that week. Across the river, the Metropolitan Detention Center in Brooklyn had the worst: only 0.3 percent of about 8,000 letters and other pieces of mail were read by the four mailroom staff, the review found.

Because of the huge volume of incoming mail, looking for suspicious correspondence is like searching for a needle in a haystack, Bureau of Prisons Director Harley G. Lappin told inspectors.

Still, the agency agreed with most of the review's recommendations to improve the system, including putting tracking systems in place by December 2007 to ensure all high-risk inmate mail is read and analyzed.