The Boston Marathon bombing suspects used "relatively sophisticated" bombs with fuses made from Christmas lights and remote-control detonators made from model car parts, federal prosecutors said in court documents filed Wednesday.
Prosecutors also said that Tamerlan and Dzhokhar Tsarnaev used fine black powder from firecrackers as fuel for the bombs and, since none was found in searches of their homes and cars, investigators worried they had help.
The prosecutors were arguing against a defense motion to throw out Dzhokhar Tsarnaev's statements to FBI agents because he was questioned without a lawyer.
"In light of the history of coordinated terrorist attacks (and planned attacks) such as the ones in Mumbai, India, Times Square, the New York subway system, and on September 11, the FBI had a duty to be investigate whether any additional attacks were imminent," the filing said. "Interviewing Tsarnaev as soon as possible was therefore essential to protect the public from possible harm."
Prosecutors say the bombs, comments Tsarnaev and his brother, Tamerlan, made to a carjacking victim that they might explode more bombs in New York and a note Tsarnaev wrote in a boat where he was captured made it imperative to know if there was a continuing terror threat before informing him of his rights, an allowed exception.
They say Tsarnaev said he and his brother acted alone and there were no more bombs.
Dzhokhar Tsarnaev, 20, has pleaded not guilty to several federal charges. Prosecutors allege he and his brother planted two pressure cooker bombs near the marathon's finish line, killing three people and injuring more than 260 others. His brother was killed during a gunbattle with police on April 19, 2013, four days after the marathon bombing.
Prosecutors said Tsarnaev explained his motive in the note he left in the boat. It said the U.S. government was killing innocent civilians. "I can't stand to see such evil go unpunished," the note said in part. "We Muslims are one body, you hurt one you hurt us all." It ended, "Stop killing our innocent people and we will stop."
This month, lawyers for Dzhokhar Tsarnaev argued the statements he made after his arrest should be thrown out because he was questioned for 36 hours in a hospital room while suffering from gunshot wounds and without being told his rights. They said the questioning continued "despite the fact that he quickly allayed concerns about any continuing threat to public safety, repeatedly asked for a lawyer, and begged to rest." They said his treatment included painkillers that impaired his judgment.
Prosecutors, though, said that before agents questioned Tsarnaev a nurse told them he had no brain injuries and his medication would not "inhibit his mental faculties." They denied Tsarnaev was coerced or mistreated, saying they waited for his condition to improve before interrogating him and gave him rest breaks. They said they don't need Tsarnaev's statements to present their case against him but reserve the right to use them to rebut any inconsistent testimony.
In separate filings Wednesday, prosecutors also rejected defense motions that the federal death penalty should be declared unconstitutional and that some aggravating factors they have cited in seeking the death penalty should be thrown out.
They said "betrayal of the United States" by Tsarnaev as a U.S. citizen is a permissible factor but they would modify it to delete a reference to his status as a naturalized citizen. They said they propose to note instead that he violated the oath of allegiance he took in becoming a citizen.
The Associated Press contributed to this report.