How did the Tsarnaevs build the bombs that killed three and injured hundreds? Overnight, investigators may have found a key stash of evidence dumped in a Boston suburb — fireworks tossed out with their gunpowder removed:

Overnight, federal agents and police discovered what could be fresh evidence of how the men believed responsible for the twin detonations at the Boston Marathon finish line constructed their bombs.

Agents uncovered a stash of firework shells, some of which appeared to have been emptied of their gunpowder. The cache was discovered in a used clothing collection bin in a grocery store parking lot in Watertown.

“It was a device that looks like a big firecracker … that had a wick in it,” said Michael Tambosi, a representative of the group Planet Aid New England, which owns the clothing donation containers.

A driver for the organization first spotted the fireworks when he opened the bin. There were about half a dozen Roman candle shells in a plastic grocery bag and Tambosi said it looked like some of them had been scooped out.

It’s not yet clear that the cache has any connection to the Boston Marathon bombings.  Investigators had already found evidence that the Tsarnaevs had bought large fireworks and speculated that they had mined such items for the powder to build the bombs.  The location of this find certainly suggests a connection to the Tsarnaevs, but it could be that someone else has the same idea, too — which isn’t exactly a comforting thought.

As ABC also reports, Tsarnaev’s condition has improved enough to move him to more secure surroundings, but as I noted earlier, that doesn’t mean he’ll be cooperating.  That’s too bad, because New York police would certainly like a few answers to their questions:

Authorities say at this point they don’t believe the two had plans for any other attacks. But they also believe there is evidence that the two brothers might have been planning a trip to New York.

The man who was car-jacked by the bombers has reported he may have overheard them talking about Manhattan. Police were also scrutinizing photos posted on a Russian social network page by a friend and one time classmate of 19-year old Dzhokhar Tsarnaev. The photo showed Tsarnaev and others in Times Square a year ago.

The New York police commissioner said he had been told by investigators that the two brothers were talking about traveling to New York after the bombings.

And before Dzhokhar stopped talking, he apparently told investigators what everyone had assumed:

Seven months later, authorities say he told FBI agents that he and his brother planted bombs along the route of the revered sporting event and killed a police officer. The reason he gave, according to authorities, was that Muslims are at war with the US.