An imam on Tuesday disputed American Taliban fighter John Walker Lindh's claim that the school of Islam to which he belongs requires him to perform his ritual daily prayers as part of a group, even though he's in prison.

Ammar Amonette, who leads a mosque in Richmond, Va., and who like Lindh, converted to Islam, testified that the Hanbali school to which they both adhere stresses that Muslims should pray in a congregation, but he said believes can be excused from their religious obligations if important work or some pressing reason prevents them from fulfilling them.

"The congregation is not considered essential by any of the four Islamic schools of jurisprudence," Amonette testified. There could, however, be "deviant" schools of belief, he said.

Lindh, 31, monitored the second day of the trial over his lawsuit challenging the prison prayer policy via a closed-circuit video conference. He testified on Monday at the outset of the trial.

Lindh is serving a 20-year sentence for aiding the Taliban during the 2001 U.S.-led invasion of Afghanistan at a tightly controlled federal prison unit in Terre Haute. He and the American Civil Liberties Union of Indiana, which is representing him, contend that the group prayer restriction flouts a 1993 law restraining the government from curtailing religious expression without showing a compelling reason to do so.

The government says it can't allow the daily prayers to be conducted in a group because it would pose a security risk. Prison system officials testified that the U.S. Bureau of Prisons doesn't have enough Muslim chaplains or enough guards to supervise daily group prayers, as its policy requires. In Lindh's unit, where the majority of inmates are Muslim, group prayers are generally led by a prisoner chosen by other inmates.

Earlier, Judge Jane Magnus Stinson sharply questioned the Terre Haute prison's supervisory chaplain about why officials restrict group prayer but don't restrict other group activities, such as meals and game-playing.

"You would need supervision because you don't know what they might be doing," David Holston, replied hesitantly.

"That's true no matter what they're doing," Stinson said.

Under questioning by deputy U.S. attorney William McCoskey, Holston said that religious gatherings led by inmates raise a special security concern because one inmate is assuming authority over the others and could incite them to violence.

"There is the potential for radicalization, the type of teaching that could pose a security threat to an institution," Holston said.

There are only 14 Muslim chaplains to serve more than 100 federal prisons, making it impossible to conduct supervised daily group prayer, the U.S. Bureau of Prisons' chief chaplain testified Monday.

Lindh is one of 24 Muslims among the 43 inmates in the Communications Management Unit at the prison in western Indiana.

According to court documents, Muslims in the tightly controlled unit are allowed to pray together only once a week, except during the Islamic holy month of Ramadan. Other faiths' gatherings are also limited. At other times, they must pray alone in their individual cells.

Prisoners in Lindh's unit are under open and covert audio and video surveillance, and except for talks with their attorneys, all of their phone calls are monitored. Prisoners aren't allowed to touch family members during tightly controlled visits. Without such strong security, the government claims, inmates would be able to conspire with outsiders to commit terrorist or criminal acts.

The lawsuit was originally filed in 2009 by two Muslim inmates in the unit. Lindh joined the lawsuit in 2010, and the case has drawn far more attention since then. The other plaintiffs have dropped out as they were released from prison or transferred to other units.

In 2001, Lindh was captured in Afghanistan by U.S. troops and accused of fighting for the Taliban. Raised Catholic, the California native was 12 when he saw the movie "Malcolm X" and became interested in Islam. He converted to Islam at age 16. Walker told Newsweek after his capture that he had entered Afghanistan to help the Taliban build a "pure Islamic state."

In 2002, Lindh pleaded guilty to supplying services to the now-defunct Taliban government and carrying explosives for them. He had been charged with conspiring to kill Americans and support terrorists, but those charges were dropped in a plea agreement. He was transferred to the Terre Haute prison in 2007.He is eligible for release in 2019.