Chechen-origin teenager Dzhokhar Tsarnaev apparently used his cell phone to blow up the pressure cooker bomb at the Boston Marathon that killed three people and wounded nearly 200 others, according to a criminal complaint filed by the FBI in a US court.
In the complaint, the FBI alleges that the suspect appeared calm while there was chaos all around after the blasts at the finish line of the marathon on April 15.
The FBI filed the complaint against 19-year-old Dzhokhar, as he was produced before a district court which temporarily was his hospital room where he is being treated of injuries.
The court fixed May 30 for Dzhokhar's first hearing in Massachusetts District court.
Dzhokhar is charged with conspiring to use "weapon of mass destruction" and faces death penalty if convicted.
In the 10-page complaint, FBI special agent Daniel Geneck gave the sequence of events and sufficient information to establish the requisite probable cause. "It does not include each and every fact known to me," he said.
Based on the information obtained from the security cameras, Geneck said, the two suspect - Tamerlan Tsarnaev, 26, and Dzhokhar Tsarnaev - were first spotted near the bombing site 11 minutes before the first bomb explosion.
While Tamerlan was wearing a dark-colored baseball cap, sunglasses, a white shirt, dark coat and tan pants, his brother was wearing a white baseball cap backwards, a gray hooded sweatshirt, a lightweight black jacket, and dark pants.
Thereafter, the two were seen moving towards the Marathon finish line, which was occupied by thousands of people, which is reflected through various security cameras.
At some point Dzhokhar appears to look at his phone, which is held at approximately waist level, and may be manipulating the phone, Geneck said.
"Approximately 30 seconds before the first explosion, he lifts his phone to his ear as if he is speaking on his cell phone, and keeps it there for approximately 18 seconds. A few seconds after he finishes the call, the large crowd of people around him can be seen reacting to the first explosion," the complaint said.
"Virtually every head turns to the east (towards the finish line) and stares in that direction in apparent bewilderment and alarm," Geneck said, adding that Dzhokhar, virtually alone among the individuals in front of the restaurant, appears calm.