Fire damage in the main compound villa in which Ambassador J. Christopher Stevens died in 2012 in the U.S. Consulate in Benghazi, Libya. (Michael Birnbaum/The Washington Post)

A U.S. judge declared a mistrial Monday after a federal jury convicted a second Libyan man of conspiracy in the deadly 2012 attacks on U.S. facilities in Benghazi, Libya, but deadlocked on 15 of 17 counts in connection with the deaths of U.S. Ambassador J. Christopher Stevens and three other Americans.

U.S. District Judge Christopher R. “Casey” Cooper found jurors hung and dismissed them after they reported themselves unable “to come to unanimous agreement on any of the remaining counts” in a note sent at 11:27 a.m.

Cooper last Thursday asked jurors to return after a weekend break following their partial verdict, in which they found Mustafa al-Imam, 47, guilty on one count each of conspiracy to provide material support to terrorists and maliciously destroying government property in overnight attacks that began Sept. 11, 2012, on a U.S. diplomatic mission and nearby CIA post.

The dismissed counts included the most serious charges of murder and attempted murder against the Americans, and also counts involving a second round of attacks on the nearby secret CIA annex in which two contractor security agents were killed and another CIA and State Department security officer injured.

In a statement, senior Justice Department and FBI officials did not announce whether the government would retry Imam, but Attorney General for National Security John C. Demers said, “We will never forget those we lost in Benghazi on Sept. 11, 2012 — Tyrone Woods, Sean Smith, Glen Doherty, and Ambassador Christopher Stevens. And we will not rest in our pursuit of the terrorists who attacked our facilities and killed these four courageous Americans — they must be held accountable for their crimes.”

Ambassador J. Christopher Stevens in the lobby of the Tibesti Hotel in Benghazi, Libya, in April 2011. (Ben Curtis/AP)

The results of Imam’s four-week trial largely tracked the outcome of a separate jury in November 2017 that found accused ringleader and Libyan militia leader Ahmed Abu Khattala, 47, guilty of four of 18 counts but not directly responsible for the deaths of Stevens, State Department communications aide Sean Smith and CIA security contractors Glen Doherty and Tyrone Woods.

Abu Khattala is serving a ­22-year prison sentence handed down by Cooper, who presided over both trials.

Stevens was the first U.S. ambassador killed while in the performance of his duties in nearly 40 years.

Imam’s capture was ordered by President Trump, and his trial in civilian court marked the first of a foreign terrorism suspect captured abroad during his administration.

Fighters loyal to the Government of National Accord gesture to a fellow fighter in the al-Sawani area south of the Libyan capital of Tripoli during clashes against forces loyal to strongman Khalifa Haftar on June 13. (Mahmud Turkia/AFP/Getty Images)

Imam faces maximum penalties of up to 15 years for conspiracy and 20 years for destruction of property.

After getting the split verdict in the initial Benghazi trial, the government switched out prosecution teams and delivered a more streamlined case against Imam in which it sought to pin accountability not only for the assault on the diplomatic mission but also for a second round of attacks hours later on a secret nearby CIA annex in which Woods and Doherty were killed in a rooftop mortar strike.