NEW YORK — Three men, including one already jailed in the United States, were accused by U.S. authorities Tuesday of being Iran-supported operatives who plotted nearly 20 attacks on Jewish communities in Europe and the U.S. as well as a planned assault on a British airbase used by the U.S. Air Force.

Charges against Mohammad Baqer Saad Dawood al-Saadi, a dual Iranian-Iraqi national; Mohsen Mohajeri, an Iranian national; and Mohammad Ali al-Hassani, an Iraqi national, were outlined in a superseding indictment unsealed in Manhattan federal court. Only al-Saadi is in custody.

U.S. Attorney Jamie McDonald in New York said in a news release that al-Saadi, who was arrested in the spring, continued to coordinate with his co-defendants in the days leading up to the attempted September terrorist plot at a British Royal Air Force Base in Fairford, United Kingdom, where U.S. bombers and troops had been stationed.

“These defendants helped orchestrate a campaign of terror on behalf of terrorist organizations affiliated with Iran that targeted Jewish communities in Europe and the United States, and the British military base, RAF Fairford, housing American service members,” McDonald said. “The threat presented by Iran and Iran-backed terrorist groups to America and our allies is not hypothetical — it is very real.”

Attorney General Todd Blanche said: “Anyone who plans a terrorist plot on a military base housing American servicemembers will be prosecuted by the United States government.”

The new indictment offers fresh insight into an episode that has been shrouded in mystery for more than a month after five men were detained after midnight on Sept. 27 on suspected explosives and terrorism offenses after three suspicious vans were spotted traveling toward the base in western England.

A sixth suspect, a British-Iranian dual national, was arrested several days later. All the men were released on bail.

British authorities linked the incident to Tehran, and though Iran denied involvement, the arrests prompted the U.S. to withdraw its bombers that had been stationed there. The bombers were redeployed to their home bases in the United States.

RAF Fairford, located 100 miles west of London, has been used by American bombers during decades of conflicts in the Middle East.

According to the indictment, al-Saadi used coded language in communications with relatives from jail to direct that money be passed along to others who could carry out attacks.

The indictment alleged that the three men conspired together to use a network of contacts in the United Kingdom to commit terrorist attacks on behalf of an Iran-aligned terrorist network, which included the Islamic Revolutionary Guard Corps and Kata’ib Hizballah.

In June, al-Saadi pleaded not guilty to charges that he plotted at least 18 attacks in Europe in retaliation for the U.S. and Israel’s war with Iran. He called himself a “prisoner of war.” He has been held without bail since. His lawyers did not immediately respond to requests for comment Tuesday on the latest development.

Both Kata’ib Hizballah and Iran’s Islamic Revolutionary Guard Corps have been designated by the U.S. government as foreign terrorist organizations. U.S. prosecutors say Al-Saadi was a Kata’ib Hizballah commander.