But in Wednesday’s filing, prosecutors wrote that FBI agents had grounds to question Tsarnaev at the time, since they had to determine whether any accomplices were still at large who might have posed a threat.

The filing in US District Court in Boston came in response to a motion from Tsarnaev’s defense lawyers to suppress statements he made to FBI agents while he was being treated in a Boston hospital after his arrest. The defense contends that Tsarnaev was interrogated at length despite repeatedly asking for a lawyer.

Dzhokhar Tsarnaev and his now-deceased brother used parts from Christmas lights and model cars to build the bombs that they allegedly detonated near the finish line of the Boston Marathon, federal prosecutors said in a court filing Wednesday.


“The Marathon bombs were constructed using improvised fuses made from Christmas lights and improvised, remote-control detonators fashioned from model car parts,” prosecutors wrote. “These relatively sophisticated devices would have been difficult for the Tsarnaevs to fabricate successfully without training or assistance from others.”

Also, prosecutors said, Tsarnaev and his brother, Tamerlan, appear to have crushed and emptied fireworks containing black powder to make explosive fuel for the bombs.

That powder is a fine material, and if the Tsarnaevs had built the bombs themselves, it was reasonable to expect traces of the powder would be found where they worked on the explosives, prosecutors said.

Related: 102 hours in pursuit of Marathon suspects

“Yet searches of the Tsarnaevs’ residences, three vehicles, and other locations associated with them yielded virtually no traces of black powder, again strongly suggesting that others had built, or at least helped the Tsarnaevs build, the bombs and thus might have built more,” prosecutors wrote.

Dzhokhar Tsarnaev’s lawyers had not responded to the filing by Wednesday evening. Tamerlan Tsarnaev was killed in a confrontation with police in Watertown days after the April 15, 2013, bombings.


Dzhokhar Tsarnaev faces several charges that could bring the death penalty.

The blasts killed three people, including a child, and wounded more than 260.

“In short, the facts and circumstances known to law enforcement at the time they interviewed Tsarnaev provided ample reason to believe that the Tsarnaevs did not act alone,” prosecutors wrote in the filing.

No one else has been charged with participating in the bombings, and prosecutors wrote Wednesday that Tsarnaev admitted his own role in the blasts under questioning, but “steadfastly denied” that anyone else was involved. Authorities have not said publicly if they are seeking additional suspects in the bombing itself.

Three of Tsarnaev’s friends face charges of obstruction of justice and lying to the FBI in allegedly removing items from his college dormitory room after the blasts and lying about it to investigators.

Graphic: Timeline of the Marathon attack and manhunt

The filing also stated that Tsarnaev ran over his brother in a vehicle during the confrontation in Watertown and later wrote about him in a note, which he scrawled in a boat where he was hiding during a manhunt that ended with his arrest.

“I’m jealous of my brother who ha[s] [re]ceived the reward of jannutul Firdaus (inshallah) before me,” Tsarnaev allegedly wrote, referring to the Islamic conception of the highest level of Paradise.

“I do not mourn because his soul is very much alive. I ask Allah to . . . allow me to return to him and be among all the righteous people in the highest levels of heaven.”


Related: Excerpts from Tsarnaev’s note

Tsarnaev, prosecutors said, went on to decry the US government for killing innocent civilians before writing, “We are promised victory, and we will surely get it. Now I don’t like killing innocent people — it is forbidden in Islam but due to said [UI] it is allowed. All credit goes [UI]. Stop killing our innocent people and we will stop.”

It was not clear on Wednesday who or what “UI” stood for, or whether Tsarnaev wrote the letters or if prosecutors inserted them in the court filing as a substitution for his words.

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Travis Andersen can be reached at travis.andersen@globe.com.