Attorneys for Dzhokhar Tsarnaev, the alleged Boston Marathon bomber, asked a federal judge to ease special restrictions that have been placed on him at the prison in Fort Devens, saying they have unduly left him in harsh isolation while preventing proper communication with his family and his legal team.

The attorneys argued that US Attorney General Eric H. Holder J. had no basis to order the Special Administrative Measures for Tsarnaev in late August, more than four months after his arrest, with no evidence that Tsarnaev was a further threat of violence or to security.

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“The government has not alleged that Mr. Tsarnaev has done or said anything since his arrest to commit violence, incite violence, or engage in communications that pose a security threat,” the lawyers argued in a courting filing Wednesday, calling the measures “unlawful and unwarranted.”

Federal prosecutors have not yet responded to the court filing.

Tsarnaev, now 20, faces 30 charges related to the April 15 bombing that killed three people and injured more than 260, including several terrorism charges that carry the possibility of the death penalty. Authorities said he also allegedly killed MIT police officer Sean Collier. His brother and alleged accomplice, Tamerlan Tsarnaev, was killed in a shootout with police in Watertown and after he was run over by a car Dzokhar Tsarnaev was driving.

In the first stage of the legal proceedings, Holder, with input from local prosecutors and the defense attorneys, is considering whether to seek the death penalty against the younger brother.

But lawyers for Tsarnaev, saying their access to him is vital to build a case against the death penalty, say that the Special Administrative Measures have unduly limited communications with him in violation of his constitutional rights.

The special measures limit who from the defense team can visit or communicate with Tsarnaev, and what information they can share with him or others. The lawyers face penalties for violating any of the measures, what they called a “chilling” effect on their judgment making. In one instance, they noted, they could not show Tsarnaev photos of family members.

The measures also restrict Tsarnaev’s communications with the outside world to only immediate family members, and those communications are limited as well. Tsarnaev also has no access to any media, and cannot communicate with other inmates, including in prayer groups.

Holder, in ordering the Bureau of Prisons to enact the measures in August, at the request of US Attorney Carmen M. Ortiz of Massachusetts, said that they are “reasonably necessary to prevent the inmate from committing, soliciting, or conspiring to commit additional criminal activity,” according to records made public Wednesday.

Prosecutors argued, according to the court records, that Tsarnaev was inspired to commit the bombings by terrorist groups, he told investigators after the bombings that he hoped he inspired others, and “there is no indication that Tsarnaev’s intentions have changed since.”

They also noted that Tsarnaev’s mother taped a May 24 telephone conversation with him and played it for the press in Russia in an apparent effort to “engender sympathy” for him. Tsarnaev has “also gained widespread notoriety while incarcerated,” the prosecutors added, saying he has received nearly 1,000 pieces of unsolicited mail.

But Tsarnaev’s lawyers argue that, “While the government may not want anyone to feel sympathy for Mr. Tsarnaev, that is not a proper basis to impose (the measures).”

They argued that there is no evidence he attempted to incite others, and that the harsh prison conditions are based more on his alleged crimes than anything he has done since his arrest.

The defense team’s request drew immediate support from the American Civil Liberties Union of Massachusetts, which said in a statement that the restrictions on Tsarnaev “threaten our Constitution’s guarantees of due process and effective assistance of counsel in this and all cases.”

“Because these extreme measures are unwarranted and severely threaten the fair administration of justice, they should be vacated,” said Jess Rossman, a staff attorney with the ACLU of Massachusetts “Particularly in such a high-profile case that will be watched closely across the world, there should be no doubt about the integrity of the process or its outcome.”