Authorities investigating a massive fire on Easter Sunday that destroyed or damaged about 40 summer cottages at a Christian center said Monday they don't believe the fire was arson.

"The are no red flags to indicate it was suspicious, at this time," Fire Marshal William Degnan said.

Flames fanned by wind off Lake Winnipesaukee swept through the summer cottages at the 146-year-old Alton Bay Christian Conference Center. Degnan said the cottages were unoccupied, as the summer season hadn't begun yet at the center.

"Fortunately, it was at this time of year," Degnan said. "During the summer, it's full."

Witnesses watched as buildings burned to the ground in minutes.

"It was a huge red inferno," Emmy Pijoan, who has worked at the conference center, told the New Hampshire Union Leader. "The fire people were dwarfed by the fire. The fire trucks looked like toys compared to the fire."

Others described the earth shaking as propane tanks exploded.

"You could feel it in your chest, then you saw the shrapnel fly up in the sky," Brendan Belleau told WMUR-TV.

One firefighter was injured when a propane tank exploded; he was hospitalized in stable condition. Others were treated for smoke inhalation or exhaustion.

The center's approximately 140 cottages are close together in a network of narrow roads.

The center's Web site says its purpose is to change lives in part by "encouraging commitment to Jesus." It says it tries to accomplish this through evangelism, encouragement of believers and equipping Christians to be effective leaders.

The center, founded in 1863, is on the southeastern corner of the lake about 30 miles northeast of Concord. It also features a 70-room motel and a park for recreational vehicles.