By Daniel Trotta

(Reuters) - A trade group representing makers of artificial limbs on Tuesday promised to provide prosthetics free of cost to the estimated 20 to 25 victims of the Boston Marathon bombings who underwent amputations.

The American Orthotic and Prosthetic Association offered initial services and prosthetics not covered by insurance for patients injured in the blasts, which tore the lower limbs off some spectators near the finish line of the race on April 15.

Costs for a below-knee device average $8,000 to $12,000 each and $40,000 to $60,000 for above-knee prosthetics, said Greig Martino, a prosthetist treating bombing victims.

Three people died and 264 were injured in the blasts, which police said resulted from two homemade bombs in pressure cookers that a pair of ethnic Chechen brothers left near the race's finish line. One of the suspects died in a shootout with police and the other has been charged with crimes that could result in the death penalty if he were convicted.

The association's offer, announced on a conference call with reporters under the name Coalition to Walk and Run Again, will only cover a portion of the expected costs for amputees. Victims who lost both legs face estimated medical bills of $450,000 over the next five years, said Tom Fise, executive director of the association, citing a Department of Defense and Veterans Affairs study.

The association estimates that at least half the Boston Marathon amputees lack enough insurance to cover their prosthetic costs as some policies provide as little as $1,000 per device or only provide one artificial limb. Many prosthetics need replacing every five to seven years.

"The last thing that someone should have to worry about when they lose ... a leg is to have adequate insurance coverage for a prosthetic device," said Kendra Calhoun, president of the Amputee Coalition, an organization supporting the estimated 2 million amputees in the United States.

Other efforts are under way to help victims of the largest mass-casualty attack on U.S. soil since the attacks of September 11, 2001. The One Fund, a relief group set up to provide aid to victims, has brought in some $27.7 million in donations that it promised to pay out to victims.

Members of the American Orthotic and Prosthetic Association, including prosthetists and manufacturers, will provide the evaluation needed to design devices and treat the bombing victims, at least one of whom is a child.

"Many of these patients even today don't know what their insurance has in store for them, nor do we, so the program is about making sure the decisions to restore mobility to these patients are made as independent as we can from any considerations of what the insurance limitations may be," Fise said.

(Reporting by Daniel Trotta; Editing by Scott Malone, Toni Reinhold)