A teen-ager fatally wounded by a policeman Feb. 20 managed to write a statement before his death contradicting the officer's contention that it was an accident, an attorney for the boy's family claimed. Officer Robert R. Duty reportedly said his pistol discharged when he tripped over a pile of wood in a darkened South-Central Los Angeles alley while investigating reports of a loud radio. Manuel Diaz, 16, died nearly two weeks later. Attorney Miguel F. Garcia, who has filed a wrongful death claim against the city for the family, denied the boy was running from the officer and said evidence indicates Duty fired from an upright position, rather than while falling. The district attorney's office is investigating.