SPOKANE, Wash. -

The FBI continues to gather more evidence that ties suspect Kevin Harpham to a bomb left along a parade route on Martin Luther King Jr.?s birthday, specifically information related to the bomb?s construction.

Agents are particularly interested in who assembled the explosive that required welding equipment and some large diameter steel pipe.

Not only was the bomb extremely heavy to carry around but it's design was much more similar to a crude piece of artillery, something Harpham, a former soldier, was likely familiar with.

The project involved a lot of welding and that's one thing the FBI was looking for when it raided Harpham?s home on March 9. The search warrant, which would detail whether agents found welding equipment at his home, remains sealed.

?There's a couple of reasons we?re not seeking a court order to open the file at this point and the first is the investigation is still ongoing, and until we complete the investigation we want to keep the information we have confidential,? U.S. Attorney Mike Ormsby said.

That investigation also involves tracking down the sources of the bomb's explosive components, specifically where Harpham allegedly bought the black powder found in the bottom of the cannon along with the bag of fishing weights that would serve as projectiles. Laid on its side on a bench, the device was positioned to shower marchers with shrapnel.

Ormsby hinted that Harpham could face additional indictments in the future.

?We will make sure the investigation is completed, and there is the possibility of additional charges against this defendant, and there may be other suspects, but I think its more likely additional charges may be brought,? he said,

FBI officials say it?s possible someone with welding equipment unwittingly fabricated that cannon for Harpham, not knowing what it was. Agents would like to hear from that person.