NEW YORK – Osama bin Laden’s brother-in-law, Sulaiman Abu Ghaith, the most senior adviser to bin Laden to be tried in a civilian U.S. court since the Sept. 11 attacks, was convicted Wednesday of conspiring to kill Americans and providing material support to terrorists.

Abu Ghaith, 48, a Kuwaiti-born cleric known for his fiery oratory, had recorded impassioned speeches for bin Laden after Sept. 11, in which he praised the attacks and promised that future attacks would be carried out.

His conviction on all three counts – and the lightning speed from his arrest to verdict – would seem to serve as a rejoinder to critics of the Obama administration’s efforts to try suspected terrorists in civilian court, rather than before a military tribunal.

“It was appropriate that this defendant, who publicly rejoiced over the attacks on the World Trade Center, faced trial in the shadow of where those buildings once stood,” U.S. Attorney General Eric H. Holder Jr. said in a statement.

The jury returned its verdict on its second day of deliberations in the trial, which had entered its third week in U.S. District Court in Manhattan. Abu Ghaith was convicted of conspiracy to kill Americans, for which he could face life in prison; and providing material support to terrorists, as well as conspiring to do so, counts that each carry maximum terms of 15 years.

His lawyer, Stanley L. Cohen, said later that his client was stoic and “at ease.”

“He has confidence that this is not the end but the beginning,” Cohen said. The lawyer added that there were “a number of compelling issues” for appeal.

Abu Ghaith, married to bin Laden’s daughter, Fatima, was captured last year and brought to the United States. His lawyers sought to portray their client as having had a minimal role in al-Qaida, with no involvement or advance knowledge of any terrorist plot.

But the prosecution repeatedly reminded the jury, often through photographs and videos, of how close Abu Ghaith had been to bin Laden.

The prosecution, for example, showed the jury a video screen shot of Abu Ghaith, made on Sept. 12, 2001, as he sat beside bin Laden.

“Sulaiman Abu Ghaith literally sat at Osama bin Laden’s right hand,” a prosecutor, John P. Cronan, said in his closing argument.

He cited another video, made on Oct. 9, 2001, in which Abu Ghaith warned that “the storm of airplanes will not abate,” and that there were thousands of Muslim youths who were yearning for death, “just as the Americans yearn to live.”

That message, Cronan said, was meant to terrorize Americans, but also “to drive more suicide terrorists to al-Qaida.”