Charge sheet against Dzhokhar Tsarnaev contains mass of new detail and reveals how CCTV footage was used to build the case

The prosecution case against Dzhokhar Tsarnaev, detailed in cool, unemotional language over 13 pages of a federal criminal complaint, begins at 2.41pm last Monday. It is then, according to FBI special agent Daniel Genck, in whose name the complaint is filed, that the teenager and his elder brother Tamerlan are first picked up by surveillance cameras about half a block from the finish line of the Boston marathon.

A few minutes later, the complaint alleges, as hundreds of onlookers cheer the marathon finishers, the 19-year-old calmly places a knapsack on the ground and begins to use his cellphone, ending the call just seconds before the first explosion tears through the crowd. But unlike everyone around him, Dzhokhar Tsarnaev bears no sign of alarm. He walks away, seconds before a second explosion erupts where he had set down his backpack.

The account of the video evidence, which has not been released to the public, is the centrepiece of the federal case against Tsarnaev, a resident of Cambridge, Massachusetts, who was arrested last Friday after a dramatic shootout with police.

He has been charged with two federal counts: using and conspiring to use a weapon of mass destruction – an improvised explosive device – and malicious destruction of property, resulting in deaths.

The criminal complaint also makes a point of alleging that the bombings damaged "interstate and foreign commerce". Though the reference to disrupted trade sounds gratuitous, it is significant, as it allows federal prosecutors to take the lead; that, in turn, paves the way for a possible federal death penalty, which would not be possible were the case handled by Massachusetts, where capital punishment is outlawed.

The charge sheet contains a mass of new detail about the evidence the FBI says it has gathered against both Dzhokhar Tsarnaev and his elder brother Tamerlan, who died in the early hours of Friday after a frenzied exchange of gunfire with police. The document says that FBI fingerprinting of the body has produced a positive match with Tamerlan Tsarnaev, aged 26.

The charge sheet says that investigators closely studied film from a security camera set above the doorway of a restaurant, Forum, close to the site of the second blast on Boylston Street in downtown Boston.

About a minute after the two suspects were first caught on camera, Tamerlan Tsarnaev – referred to in the complaint as Bomber One – is seen detaching himself from the crowd and walking along Boylston Street towards the location of the first bombing with a large knapsack still on his back, the FBI says.

The FBI released this photo of the suspects just before the bombs went off. Photograph: FBI/AP

At 2.45pm, Bomber Two – whom the FBI alleges is Dzhokhar Tsarnaev – also detaches himself from the crowd and moves to stand directly in front of the Forum restaurant, near the metal barrier where many marathon spectators are packed. He is then shown on the footage apparently slipping his knapsack on the ground, a detail the FBI says it has checked against a still photograph taken from the opposite side of the street that shows the bag at his feet.

Dzhokhar Tsarnaev then spends the next four minutes occasionally looking into his cell phone, and once appearing to take a photo with it, the charge sheet says. "At some point he appears to look at his phone, which is held approximately waist level, and may be manipulating the phone."

He keeps the phone to his ear for 18 seconds, just before the first blast erupts. The FBI then alleges that Tsarnaev, his movements captured on video, displays highly unusual and suggestive behaviour. "A few seconds after he finishes the call, the large crowd of people around him can be seen reacting to the first explosion. Virtually every head turns to the east [towards the finish line] and stares in that direction in apparent bewilderment and alarm.

"Bomber Two, virtually alone among the individuals in front of the restaurant, appears calm. He glances to the east and then calmly but rapidly begins moving to the west, away from the direction of the finish line. He walks away without his knapsack, having left it on the ground where he had been standing. Approximately 10 seconds later, an explosion occurs in the location where Bomber Two had placed his knapsack."

The FBI alleges that from its extensive study of photographic and video evidence taken from the scene, at many different angles, there is nothing else that might have caused the second explosion other than Dzhokhar Tsarnaev's knapsack.

The court papers also reveal new details about how the two bombing suspects carjacked a black Mercedes SUV, which they used to try and make their escape before ending up in a shootout with police in the Boston suburb of Watertown. The car owner, who is not named in the complaint, told investigators that he was sitting in the vehicle when a man approached and tapped on the passenger window.

When the driver rolled down the window, the man forcefully entered the car and pointed a gun at him. "Did you hear about the Boston explosion? … I did that," the man said. The driver appears to have been incredulous, because the man then removed the magazine from his gun and showed him that it was fully loaded, then re-clipped it and said: "I am serious."

The charge sheet records that the driver of the Mercedes reported to investigators that the initial carjacker was joined by second man, and that they spoke to each other in a foreign language. The pair allegedly forced the driver to hand over $45, and tried to get more money out using his credit card at an ATM. Later, they stopped at a petrol station where the two men got out of the car. Contrary to earlier reports that they voluntarily let the driver of the stolen vehicle go, the charge sheet says that the victim managed to escape.

According to the document, at least three pressure-cooker bombs were discovered, two used in the marathon explosions, and at least one that was detonated during the final shootout with police in Watertown on Friday. The pressure cookers were all of the same brand, and contained metallic ball bearings and nails, as well as explosives, and green-coloured hobby fuse.

When the suspect was eventually captured in his hiding place in the boat, the charge sheet alleges, an ID card from University of Massachusetts at Dartmouth and credit cards were found in his pockets that all bore the name Dzhokhar Tsarnaev. A further raid on Tsarnaev's dorm room at the same university uncovered a large ignition device of the sort used to spark fireworks, and a black jacket and white hat similar to that seen worn by Bomber Two as he was caught on film shortly before the marathon explosions.

The accusations do not mention the killing of Sean Collier, an MIT police officer who was shot dead on Thursday evening at the start of what appears to be a sequence of violent events involving the bombing suspects. His death is likely to be the subject of a separate, state charge.