WASHINGTON — A former Blackwater Worldwide security guard accused of taking part in a 2007 shooting of Iraqi civilians at a Baghdad traffic circle has been indicted on a new first-degree murder charge as the case moves closer toward trial, prosecutors said Friday.

The federal grand jury indictment of Nicholas Slatten came just weeks after a judge dismissed all charges against him because of statute-of-limitation concerns.

Also Friday, the U.S. Attorney’s Office in Washington, D.C., asked that Slatten go on trial with the other three defendants in the shootings, a move the defense may seek to avoid. A joint trial is often viewed as an advantage for prosecutors.

In court papers, the prosecution said a joint trial is especially appropriate because the criminal conduct charged in the indictments was part of a single event. In addition, says the court filing, it would be grossly inefficient to try the case twice, inasmuch as the trial will involve 70 to 80 witnesses, many of whom live overseas.

The trial arising from the Sept. 16, 2007, shootings is scheduled to start in June in Washington’s federal court and to last several months. The contractors are accused of killing 14 Iraqis and wounding more than a dozen others, shootings that inflamed anti-American sentiment in Iraq and heightened diplomatic sensitivities.

Prosecutors have portrayed Slatten as a central figure in the shootings, arguing that he fired the first shots at Nisoor Square without justification at a driver who was stopped at the traffic circle. In a statement Friday, the U.S. Attorney’s office called that killing “intentional and unprovoked.”

Slatten’s lawyer, Thomas Connolly, declined to comment Friday.

Defense attorneys for the Blackwater guards say the men were ambushed by Iraqi insurgents and returned fire in an act of self-defense.

Separate trials are often favored by defendants because the separation can narrow the range of evidence that can be introduced by prosecutors, said University of Michigan law professor Samuel Gross, who specializes in criminal procedure. In addition, a defendant may want to blame another defendant for a crime and doing so in the context of a separate trial means that the other defendant is not present to respond, said Gross.

The first-degree murder charge raises a question for the other defendants, who are charged with manslaughter. They might object to going on trial with Slatten in a first-degree murder case that underscores for a jury the seriousness of the shootings. First-degree murder carries a mandatory term of life imprisonment on conviction.

Slatten faces arraignment on Monday. The grand jury indictment was handed up Thursday and was made public Friday.

U.S. District Judge Royce Lamberth dismissed the indictment against Slatten on April 23 after a federal appeals court said the statute of limitations had lapsed before the government filed the charges against Slatten last October. The statute of limitations restricts the time in which legal proceedings may be brought. There is no time limit on bringing a case of first-degree murder.

Bringing a murder case raises the bar on getting a conviction because the government would have to prove that the shootings were premeditated. Slatten had been charged with manslaughter, attempted manslaughter and using a firearm in a crime of violence.

Prosecutors have said they plan to introduce evidence against Slatten unrelated to the Nisoor Square shootings.

According to a court filing by prosecutors in March, Slatten said he wanted to kill as many Iraqis as he could as “payback for 9/11,” and he repeatedly boasted about the number of Iraqis he had shot, including an old Iraqi woman who had a knife in her hand. That incident occurred while Slatten was in the Army, the filing stated.

In various locations in Baghdad, the court filing said, Slatten deliberately fired his weapon to draw out return fire and instigate gun battles in a manner that was inconsistent with the use of force and escalation of force policies that governed Blackwater personnel in Iraq.