New York's WYNC Radio reported that jurors, within an hour of starting their deliberations, sent a note to U.S. District Judge John Gleeson seeking clarification on the elements of the charges against Adis Medunjanin. Later, the jury asked for phone records in the case.
"The jury is taking it seriously. The case is complex," defense attorney Robert Gottlieb said.
In closing arguments last week, Assistant U.S. Attorney Berit Berger described the 28-year-old Bosnian-born defendant as willing "to strap a bomb to himself, walk into a New York City subway and blow it up."
Berger told jurors Medunjanin was assigned the suicide mission by al-Qaida while on a 2008 visit to Pakistan with two high school friends, Najibullah Zazi and Zarein Ahmedzay, and that the three conspired to detonate homemade explosives in subways in 2009.
Gottlieb called the three men "immature, naive and clueless" when they set out for Afghanistan and said they "wanted to fulfill some romantic version of jihad" to retaliate for what they perceived as oppression of Muslims in the United States.
Gottlieb contended Medunjanin never intended to kill anyone, at home or overseas.
Medunjanin pleaded not guilty to conspiring to use weapons of mass destruction and conspiring to provide material support to al-Qaida. He faces life in prison if convicted.
His co-conspirators pleaded guilty to planning an attack and are awaiting sentencing. Both testified at Medunjanin's trial.
The FBI and the New York Police Department thwarted the bomb plot, days before it was to be executed, in September 2009.