The shooting at the Army base in Texas killed 13 people and wounded more than two dozen others.
The lawsuit alleging negligence by the government says that the Defense Department is avoiding legal and financial responsibility for the killings by referring to the shootings as "workplace violence" rather than as a terrorist attack.
The group also is suing the estate of Anwar al-Awlaki, a radical U.S.-born Islamic cleric who the victims say inspired the Army psychiatrist, Maj. Nidal Hasan, to carry out the attack. The two men exchanged emails before the shootings.
