Em Universo Cinematográfico Marvel :
Howard and Maria Stark were killed in an automobile “accident” secretly arranged by HYDRA on Long Island in 1991, to remove him as an obstacle in their plans.
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Winter Soldier killed Howard Stark and his wife to steal the super soldier serum and made it look like a car accident.
Nos quadrinhos, por Wikipédia
In the comics, Howard and his wife Maria died in a car crash, as the result of faulty brakes arranged by either business competitors Republic Oil & Gas (later Roxxon Energy Corporation) or by the V-Battalion.