Dzhokhar allegedly told his interrogators the weekend after he was arrested that he and his brother originally intended to set off their bombs somewhere on the Fourth of July, officials said.

But, according to the officials, Dzhokhar said that when the brothers finished the bombs sooner than they expected, they decided to stage the attack earlier, on Patriot’s Day, a proud holiday in Boston that hosts the marathon, a Red Sox game and other festivities.

Officials say the surviving suspect also said the bombs were built in the house of his brother, Tamerlan. Authorities had previously disclosed that explosive residue had been found there.

And a backpack that officials say was snatched from Dzhokhar Tsarnaev’s dorm room then thrown away by three of his friends before being recovered is providing even some clues on the bombs.