Prosecutors are accusing Bell Helicopter Textron Inc. and some of its employees of helping to foil a 31/2-year Justice Department fraud investigation by refusing to cooperate and invoking the Fifth Amendment. The inquiry into alleged massive overcharges for military spare parts was further hindered because Army representatives at Bell's factory in Fort Worth, Texas, allowed the company to destroy important financial records for years, according to a prosecution memo made public Tuesday. The prosecution summary was dated March 11, the day after the Army announced a settlement in which Bell agreed to return $88 million to the government.