NBC Nightly News with Brian Williams

And now to our NBC News exclusive tonight. This past weekend marked 12 years since the first World Trade Center bombing. Six people were killed that day. Three of the convicted terrorists in that case were locked away in a so-called supermax maximum security prison in Colorado, where prisoners have few freedoms. Apparently, though, these prisoners have been free to write articles and letters praising bin Laden and preaching a message of death to the US. NBC News senior investigative correspondent Lisa Myers tonight on an apparent and astounding breach of prison security.

LISA MYERS reporting:  It was 12:18, lunchtime, when the van exploded. The massive bomb rattled the World Trade Center, leaving a giant crater in the underground garage. Six were killed, more than 1,000 wounded. At that time, it was the worst act of terrorism ever committed on American soil. These three Islamic extremists were among those convicted, each sentenced to more than 100 years in prison. Former prosecutor Andy McCarthy convicted others involved in this attack.

Mr. ANDREW McCARTHY (Former Assistant United States Attorney): It’s difficult to imagine people who are more evil or inclined to do, you know, more mass homicide.

MYERS: So the men were sent to America’s most secure federal prisons, eventually to supermax in Colorado, supposedly unable to do further harm.

Or so we thought. Letters and articles obtained by NBC News show that, while behind bars, the bombers continued their terrorist activities, writing letters to other suspected terrorists and brazenly praising Osama bin Laden in Arabic newspapers.

According to confidential Spanish court documents obtained by NBC, at least 14 letters went back and forth between the World Trade Center bombers and a Spanish terror cell. February 2003: Trade center bomber Mohammed Salameh writes, “Oh, God! Make us live with happiness, make us die as martyrs. May we be united on the day of judgment.” The recipient, Mohamed Achraf, later allegedly led a plot to blow up the National Justice Building in Madrid and is awaiting trial. July 2002: A letter Salameh sent from prison is published in the Al-Quds newspaper proclaiming “Osama bin Laden is my hero of this generation.”

Mr. McCARTHY: He was exhorting acts of terrorism and helping recruit would-be terrorists to the jihad.

MYERS: From inside an American prison?

Mr. McCARTHY: From inside an American prison.

MYERS: The letters to the bombers spoke of the need to “terminate the infidels” and said “the Muslims don’t have any option other than jihad.” Among those corresponding, this man, charged with recruiting suicide operatives in Spain. Spanish officials accuse him of using letters to and from the US bombers as a recruiting tool. All this, while the Bureau of Prisons reassured the public terrorists were under control.

Mr. HARLEY LAPPIN (Federal Bureau of Prisons Director): We have been managing inmates with ties to terrorism for over a decade by confining them in secure conditions and monitoring their communications closely.

MYERS: Today, federal prison officials refused to comment directly on what other law enforcement officials call a horrible lapse, saying only that inmates’ letters are “monitored” and “inspected.” So how did this happen? Federal officials tell NBC that the Justice Department failed to restrict communications to and from the three bombers because key officials didn’t consider them all that dangerous. Michael Macko lost his father, Bill, in the Trade Center bombing and attended the 12th anniversary memorial this weekend.

Mr. MICHAEL MACKO: If they were encouraging acts of terrorism internationally, how do we know they’re not encouraging other acts of–of terror right here on–on US soil?

MYERS: Among the many questions now being scrutinized by the Justice Department. Lisa Myers, NBC News, Washington.