Lawyers for Boston Marathon bombing suspect Dzhokhar Tsarnaev (joh-HAHR' tsahr-NEYE'-ehv) say a remark he made to his sister during a prison visit was meant to be funny and wasn't incriminating, as prosecutors have suggested.

In court documents filed Wednesday, Tsarnaev's lawyers responded to a claim by prosecutors that Tsarnaev made a remark "to his detriment."

Neither side has revealed what Tsarnaev said. An FBI agent who was in the room because of special prison restrictions placed on Tsarnaev overheard it.

Tsarnaev's lawyers say the comment was "jocular" or "gently mocking" toward the security concerns and prison restrictions. They say it didn't relate to the charges against him.

Tsarnaev faces the possibility of the death penalty in the April 15 bombing, which killed three people and injured more than 260.