Jose Padilla. Richard Reid. Both are petty criminals-turned-terror suspects and both were easy targets for prison gangs who were on the prowl for new members.

Padilla, a quiet Catholic boy originally from New York, went from gang member to convict and finally, following a jailtime conversion to Islam, to terror-plot suspect, accused of scheming to detonate a radioactive dirty bomb in the U.S.

Richard Reid was born in London and later became a petty criminal who converted to Islam while living in a juvenile home. He, too, got caught up in a terrorism plan and became known as the shoe bomb suspect when he allegedly tried to set off a small bomb he'd hidden in his shoe as he sat in an airplane seat.

Both suspects have shown a susceptibility toward joining organized crime. They're exactly the sort of prisoner the jail gangs want.

"They look for the weak," said Robert Walker of gangsorus.com, a gang-member identification Web site.

The capture of homegrown terror suspect Padilla on May 8 as he arrived at Chicago's O'Hare International Airport from Pakistan has served as a reminder of the nation's enemies within — who use their American citizenship to blend into society and execute attacks.

Frequently, they join other malcontents while they're languishing in jail.

"Our prisons are stuffed full of people who have a hatred of the prison administration, a hatred of America and have nothing but time to seethe about it," said Robert Fosen, former assistant commissioner of New York state prisons.

Men like Reid and Padilla are obvious choices for Islamic fundamentalists seeking new blood to carry out terrorist attacks.

"Oftentimes they want a way to lash out or feel important. They are very likely to join groups that facilitate that anger," Fosen said. "Anti-American feelings help all sorts of gangs recruit in prison."

It might, at first, seem a big leap to go from run-of-the-mill gang member fighting turf skirmishes to Islamic fundamentalist trying to kill thousands of Americans.

But it's not as much of a stretch as it appears. Neal Pollard of the Terrorism Research Center said suspects like these are already predisposed to committing crimes.

"Prisons are likely to be breeding grounds for any criminal activity. Terrorism is no exception," said Pollard.

Though Padilla has been described by one former Chicago neighbor as "so quiet, so nice" — a boy who enjoyed basketball, did well in school and played video games with friends — he began committing crimes as a young teen, after he joined a Chicago street gang.

At age 15, he was involved in an armed robbery that led to one victim's death and was charged with aggravated battery, armed robbery and attempted armed robbery, and spent time in juvenile detention for his part in the crimes.

In Florida, he was convicted in 1991 in Sunrise on charges of aggravated assault and discharging a firearm, court records show. Padilla, who identified himself as Catholic when he was booked on those charges, served one year of supervised release, until Aug. 4, 1993.

Police said Padilla had brandished a .38-caliber revolver at another driver during a traffic encounter and later fired a shot from his car. No one was injured.

While in the Broward County jail, Padilla was accused of battery on a jail officer and resisting without violence in January 1992. He pleaded guilty and spent 10 months behind bars.

But terrorism-related crimes require a whole other level of group dedication, the kind sometimes rooted in fanatical faith.

"To engage in a terrorist campaign and become a member of Al Qaeda, where you're not only willing to commit a crime but commit suicide in the process, that is beyond your normal crime," Pollard said.

In recent years, as the prison population has soared, so has the work of jail ministry groups. Experts say Muslim ministries have been especially effective, though most only want to turn bad guys into good guys.

But some bad guys stay bad. When they go free, those prisoners can wind up falling in with fundamentalist terror groups, who are only too happy to have new recruits.

Being behind bars not only contributes to hard feelings, but it can also provide a harbor for terrorists to act against the United States within its own borders.

Sheik Omar Abdel-Rahman, serving a life sentence in New York for plotting to blow up landmarks, is accused of sending messages from prison through visiting attorneys that directed terrorist acts to followers.

Officials at the U.S. Marshals Service, responsible for guarding accused American Taliban John Walker Lindh and Zacarias Moussaoui, accused of conspiracy in the Sept. 11 attacks, say they are taking extra precautions to make sure no criminal contacts occur.

Prisons are adjusting to the nation's need to keep track of dissidents.

In New York, a senior prison official said the prisons have asked Islamic religious groups whether they support terrorist groups. The official, speaking on condition of anonymity, said one group has been barred from ministering at the prisons. The official declined to identify the group.

In Florida, prison officials said the Sept. 11 terrorist attacks have not led to new policies — security was already high — but they raised awareness that some religious groups could be linked to illegal activities.

"We examine and look at every group, religious included, as a possible threat to security of the institution, to the staff and to inmates," said Sterling Ivy, spokesman for the Florida prison system. "We are constantly analyzing all aspects of religion in prison, but at the same time we must maintain a sense of religious freedom based on the Constitution."

Alex Taylor, chief chaplain for Florida prisons said, "After Sept. 11, a lot of prisoners tried to tell us that this is what the Muslim prison groups were preaching. It was looked into and there were no sustainable accusations."

Authorities have also been monitoring contacts between American extremists and foreign terrorist groups to make sure they don't collaborate on attacks. These include neo-Nazis, white supremacists and Black Muslim factions.

All three of those groups have a history of recruiting in prison. In the 1980s, several groups of skinheads used prisons in Los Angeles as a recruiting ground. In the 1960s, the Black Panthers reached out to prisoners, offering legal advice and membership.

Under federal law, prisons must allow inmates access to religious leaders and texts. But they don't have to allow religious groups that advocate violence to minister.

"If they stir up conflict, security hears about it right away and it is terminated," Taylor said.

Jennifer Wayton, a researcher at Texas Tech University who studies released prisoners, said security alone won't solve the problem.

"Just trying to keep terrorist groups or criminal groups out of prison won't be effective," Wayton said. "We've tried that for years and gotten nowhere.

"We must provide a better chance for people in prison to be reintegrated into society, or they will go back to crime and they will be ripe possibilities for all sorts of criminal groups."

Fox News' Heather Nauert, Catherine Donaldson-Evans and The Associated Press contributed to this report.