The Investigators: Who is behind Chelsea explosion?

CHELSEA, Manhattan (WABC) -- The small bomb that exploded in a trash can in Seaside Park, New Jersey seemed at first to be more like a prank taken too far, but then hours later a larger explosion that injured people in Chelsea.

Now, that Seaside bomb takes on a more troubling significance - are they connected; both used cell phone detonators, both exploding in trash cans. It is just one of a myriad of angles investigators are working through to try to find who's behind these blasts.

"The trade-craft worked because the bomb went off," said former NYPD Commissioner Ray Kelly.

Kelly says the explosion in Chelsea and a pipe-bomb blast earlier in the day at Seaside Park, New Jersey are hard to see as just coincidental.

"People want to reassure the public, and I understand that, but it's much too early to say whether or not there's a connection," Kelly added.

However, the current NYPD Commissioner, James O'Neill seemed to rule that out.

"We have to see if there's a connection, and at this point, there doesn't appear to be one," O'Neill said.

There is also a homemade pressure cooker with wires and a cell phone found on 27th Street just blocks from the 23rd Street blast.

"We're going to get a treasure trove of evidence from this," says Former Deputy MTA Security Director Nick Casale. The pressure cooker could have fingerprints or other DNA. The homemade device is so similar to the ones that killed three people at the 2013 Boston Marathon, that this former NYPD Detective says their use Saturday night in Chelsea can only lead to one conclusion.

"That's starting to tell me it's organized, and that this has the intent of inflicting terror on the people. Put a period there - this is terrorism," added Casale.

The last bombing in Manhattan occurred eight years ago, when a crude explosive went off outside an Army Recruiting Center in Times Square. No one was injured, but the bomber got away. Despite bomb pieces and vide, the NYPD and FBI have yet to catch him. Now they have another bomber they need to find before he strikes again.

"We'll look at individuals, at associations, look at social networks, we'll look at incoming tips and leads," adds FBI Deputy Director Bill Sweeney.

"I am concerned! I know we're going to find out who did this, and they'll be brought to justice," added NYPD Commissioner O'Neill.

Eyewitness News has learned that the explosives used in the Chelsea bomb was a mixture of ammonium and nitrate called 'Tannerite'. Tannerite is often used in target practice intended to ignite when shot at by a gun. What is troubling is that it is legal in most states, and can be ordered on the internet and mailed to your home.