The United States government (plaintiff) prosecuted Hazen Shaw (defendant) for possessing an unregistered sawed-off shotgun shorter than 18 inches in length, in violation of 26 U.S.C. §§ 5861 and 5871. Each statute contained a scienter element but no express mens rea element. The trial evidence showed that state troopers responded to a report that a man was firing shots in a residential area. The man sped away in his car and later tried to elude a roadblock by ramming a trooper's vehicle. Troopers finally trapped the man, and as they approached his car, they heard him unload a pump-action shotgun. The troopers identified the man as Shaw and arrested him for eluding arrest and reckless conduct. The troopers searched the passenger compartment of Shaw's car and found a shotgun that was visibly sawed-off, a spent shell, and ammunition in Shaw's pack and on his hunting belt. The shotgun's interior barrel length was 16.25 inches, and the barrel's exterior appearance was even shorter. A state police expert could tell at first glance that the barrel was too short. Troopers found a longer sawed-off shotgun with a different gauge in the car's trunk. Additional evidence indicated that Shaw had been sitting in his car and shooting at game through the car's open window. Shaw contended that the government failed to prove that he knew the barrel was too short, and moved to dismiss the case. The trial judge denied Shaw's motion and the jury convicted him. Shaw appealed to the United States Court of Appeals for the First Circuit.