Fourteen years after four men were convicted in a trial stemming from the deadly 1998 bombings of two United States Embassies in East Africa, a third trial in that attack began in Manhattan on Thursday.

The defendant, Khaled al-Fawwaz, is one of the earliest and most senior alleged members of Al Qaeda to be tried in the United States. In an opening statement at the trial, a federal prosecutor, Nicholas J. Lewin, depicted Mr. Fawwaz as a loyal and trusted operative who had worked for Osama bin Laden in the 1990s when Al Qaeda was a small, tight-knit group.

He had run a Qaeda training camp in Afghanistan, and then helped to lead a terror cell in Nairobi, Kenya, Mr. Lewin said. Later, Mr. Fawwaz became “Bin Laden’s man in London,” pretending to live peacefully as a Saudi dissident but actually helping Al Qaeda “broadcast its message to the world,” Mr. Lewin told the jury.

He said that, Mr. Fawwaz helped to publicize Bin Laden’s 1998 fatwa stating that Muslims should kill Americans anywhere in the world, and he vetted journalists who wanted to interview Bin Laden, who was living in the mountains of Afghanistan.