Western Iowa authorities are trying to determine the cause of Thursday’s explosion and fire, which heavily damaged the grain elevator at the Denison Seed Company. Denison Police Chief Rod Bradley says four people were inside the building when the structure exploded.None of the four were hurt. One man says he first saw the floor ripple and was then raised up several feet by the force of the blast. Seed Company General Manager David Leiting says the 81-year-old structure was in its last days.Leiting says the building was obsolete and would have been shut down at month’s end. He says the explosion, fire and its aftermath could have been much worse.Fire officials say more than 600-thousand gallons of water was used on the blaze, which is still smoldering this morning. Seven area fire departments responded to the incident.