The Honolulu Fire Department said Friday it cannot pin down exactly what caused the fireworks explosion at a Waikele storage bunker that killed five men in April.

But at least four other federal and state agencies are still investigating the tragedy, and spokesmen for family members of several victims said they are waiting to hear what the agencies say.

The explosion appears to have originated inside the front portion of the bunker, said Capt. Terry Seelig, HFD spokesman. However, "we’re not able to determine the most probable cause because there were so many possible ignition sources," Seelig said.

The cavelike bunker is leased to Donaldson Enterprises Inc., which was contracted to dispose of aerial fireworks that had been seized in 2009 and were being held as evidence. The five men killed were Donaldson employees.

The HFD investigator looked at three sets of scenarios, Seelig said. One involved possible accidental causes including weather-related activity, electrostatic buildup, sparking, cellphone use, smoking and improper fireworks handling. A second set of scenarios included operational or accidental actions by the workers in the bunker, Seelig said. Also looked at was the possibility the fireworks were set off deliberately, although investigators "feel that the explosion was unlikely malicious in nature," he said.

The Fire Department investigation has been closed, but could be reopened if new information is found, he said.

"Of course we were hoping there would be some answers here, but this is the best that we can do and we did a very thorough job," Seelig said.

Killed in the blast were Bryan "Keola" Cabalce, 25, of Wahiawa; Robert "Kevin" Freeman, 24, of Aiea; Justin Kelii, 29, of Kaneohe; Robert Leahey, 50, of Waianae; and Neil Sprankle, 24, of Aiea.

Seelig said he did not know whether the workers were preparing fireworks for demolition at another site.

Don Holmstrom, lead investigator of the Waikele incident for the U.S. Chemical Safety Board, told the Star-Advertiser days after the explosion that the tunnel was being used not just for storage, but also for preparation of fireworks for demolition elsewhere. Federal regulations pertaining to fireworks indicate prepping fireworks for disposal at a storage site is prohibited.

Jordan Lowe, resident agent-in-charge for the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives, said his agency is still investigating the incident. So too are the Chemical Safety Board, the Environmental Protection Administration and the state Health Department’s Occupational Safety and Health Division.

Jim Leahey, brother of Robert Leahey, said he was surprised to learn the Fire Department could not determine what caused the explosion.

Attorneys for the families of two other victims told the Star-Advertiser they are looking forward to seeing what conclusions are drawn by other agencies investigating the incident.

Donaldson had a license to store fireworks at the Waikele bunker, but the license expired March 31, less than 10 days before the explosion. Seelig said that detail had no bearing on the investigation.

Ryoko Donaldson, the company’s owner, declined to comment Friday on the HFD investigation.