DENVER -- The U.S. Forest Service held a memorial service Thursday for the two pilots who died fighting the Big Elk Fire between Lyons and Estes Park, Colo.

Pilots Rick Schwartz, 39, and Milt Stollack, 56 (pictured, left), were flying a 57-year-old PB4Y-2 Privateer firefighting tanker when a left wing tore off and the plane exploded in midair before it crashed into the mountainside last Thursday.

The service took place at the Jefferson County Airport tanker base where the pilots refueled each day while they were fighting the fire.

"We're talking about two brave individuals and men that did a job that they loved and cared about," said Mike Stubbs, cousin of one of the victims. "They knew that there were risks, but their sense of honor ... their sense of service and their sense of duty made them take those risks."

Brandi Stollack (pictured, left) wore her father's bomber jacket to the memorial service. She talked about her trip to the crash scene and how she and a friend of her father ran their hands over the stilled propeller blades.

"We touched the last thing that Rick and my dad touched," Stollack said at the service.

An investigation is under way to determine why the plane crashed. Investigators are looking at possible metal fatigue, which occurs when parts of the plane are stressed to the point that it breaks apart, 7NEWS reported. Most of the burned and crumpled debris, which was strewn over a large area, has been removed from the mountain.

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The fire that they fought is now 40 percent contained.

Schwartz and Stollack were among just 70 in the nation who risk their lives to help fight fires from the air.

Stollack's daughter arrived in Colorado this week to visit the crash site and to gather his belongings. She said her father and all other airtanker pilots are heroes.

"It takes a lot of courage for a tanker pilot to go out there and get in a 50-year-old airplane and put it in the air," Brandi Stollack said. "And you do your job. You do brave things. They're saving houses and lives. And they do it because they love it. They do it because they have to."

She said that Tuesday was the day that she was supposed to pick her father up at the airport in Ontario, Calif. Her father promised her that he would retire soon, as soon as a replacement could be found for him, Brandi Stollack said.

The two fallen pilots were not employed by the Forest Service but worked for a Wyoming company that contracts with the government to fly slurry-dropping planes.

Schwartz's wife is due to give birth soon, and memorial attendees brought baby items to the open house.

In lieu of flowers, the family has requested donations be made to the memorial funds for the pilots.

Rick Schwartz Memorial Fund
Wells Fargo
Attn: Jennifer Ward
PO Box 5011
Great Falls, MT 59403
Acct # 5924415614

Milt Stollak Memorial Fund
Union Bank of California
Bank # 334
11551 Foothills Blvd.
Rancho Cucamonga, CA 91730
Routing #122000496 Account 3341078552

Baby donations and sympathy cards can be sent to Liz Schwartz at:
P.O. Box 15
Ulm, MT 59485

Cards can be sent to the family of Milt Stollak at
P.O. Box 2412
Cathedral City, CA 92235