A judge has ordered two Marines to disclose whether they will testify against their former squad leader who is the first to be tried under a federal law that allows the prosecution of former combatants for war crimes.

The two Marines — Sgt. Ryan Weemer and Sgt. Jermaine Nelson — risk being jailed Friday if they refuse to obey the order to testify against Jose Luis Nazario Jr. The two have already been jailed twice for refusing to testify before a federal grand jury looking into allegations that Nazario shot and killed unarmed detainees in November 2004 in Fallujah, Iraq.

Weemer and Nelson haven't accepted an offer of immunity and therefore do not have protection against self-incrimination. Both face courts-martial on charges of unpremeditated murder and dereliction of duty for their alleged roles in the killings.

It's the latest development in the first-of-its-kind federal trial in which a civilian jury will decide whether the alleged actions of a former service member in combat violated of the rules of engagement.