The surviving Boston marathon bomb suspect has been charged with conspiring to use a weapon of mass destruction - as he and his brother are accused of bragging about the attacks to a car-jacking victim.
Dzhokhar Tsarnaev and his dead brother Tamerlan said to the motorist they abducted while on the run: "Did you hear about the Boston explosion" and "I did that", court papers revealed.
The allegations emerged after Dzhokhar, 19, was charged over the deadly attacks while in his hospital bed.
He remains in a serious condition after being detained on Friday.
He will also face a charge of malicious destruction of property resulting in death, the Justice Department said. He could face the death penalty if convicted.
Court papers also revealed Dzhokhar had apparent gunshot wounds to his head, neck, legs and hand when he was arrested at the end of a major manhunt, hours after escaping police in a shoot-out that left his 26-year-old brother dead.
Dzhokhar is being treated at Beth Israel Deaconess Medical Centre, where some of those wounded in the blasts are also being treated.
Earlier reports said he was unable to speak because of an injury to his throat, but that he was able to respond to questions from investigators by writing down the answers.
Three people died and more than 180 others were injured in the twin blasts at the marathon, the worst bombing to take place in the US since 9/11.
After Tamerlan was killed early on Friday, Dzhokhar escaped, triggering an hours-long manhunt that shut down Boston.
It ended with the teenager's capture in a boat parked in a back yard late on Friday night.
It has also emerged police chiefs believe the Tsarnaev brothers were planning further attacks.
Commissioner Ed Davis said officers found a large stockpile of weapons and more than 250 rounds of ammunition at the scene of the earlier gun battle.
"They had IEDs," he said, referring to improvised explosives devices. "They had homemade hand grenades that they were throwing at the officers.
"The scene was loaded with unexploded improvised explosive devices that actually we had to point out to the arriving officers and clear the area."
He said one IED was found in the car-jacked Mercedes the brothers had abandoned. Court papers also revealed a "large pyrotechnic" and white cap matching surveillance footage near the scene of the attacks were found in Dzhokhar's dormitory room.
Meanwhile, a lawyer for Tamerlan's wife, Katherine Russell Tsarnaev, has said US authorities have also asked to speak with his client.
Amato DeLuca said Mrs Russell Tsarnaev did not suspect her husband of anything, and nothing seemed amiss after the explosions.
The Tsarnaevs were ethnic Chechens who had been living in the US for a decade.
Dzhokhar became a US citizen in 2012, while his older brother's application was reportedly held up.
Tamerlan began posting militant videos on social media sites in recent years, and travelled to Dagestan, which borders Chechnya, in 2012. Both Russian regions host separatist rebel groups.
The brothers' social media pages appeared to express sympathy with the struggle in Chechnya, which has been ravaged by two wars since 1994 between Russia and increasingly Islamist-leaning separatist rebels.
The White House has said that as a naturalised US citizen, Tsarnaev will not be treated as an "enemy combatant", which would have involved a military tribunal. Instead he will be prosecuted through the criminal court system and is expected to appear in court on May 30.
Many in Boston have observed a moment of silence exactly a week after the first of two bombs went off near the marathon finish line.
A private funeral has been held for Krystle Campbell, a 29-year-old restaurant worker who was among the dead, on Monday.
Later in the day, a memorial service was held at Boston University for another victim, Lu Lingzi, 23, a graduate student from China.
The youngest person to die was eight-year-old Martin Richard, while police officer Sean Collier was killed during a confrontation with the suspects in the early hours of Friday.