The D.C. Council adopted the law with the goal of allowing the government to pursue charges even if it couldn't prove who possessed a gun found in a car, according to the opinion. The defendant in today's case, Antwaun Conley, was arrested after police found a loaded handgun in the center console of a car he was driving (there was also a passenger in the front seat). A jury acquitted Conley of gun possession charges–there was no physical evidence tying him to the gun–but found him guilty of the contested law.

A divided three-judge panel of the D.C. Court of Appeals found the law , which went into effect in 2009, was unconstitutional. The court said the law violated a defendant's due process rights by shifting the burden of proof to the defense and punishing behavior no ordinary citizen would believe was criminal. The third judge on the panel agreed the defendant's conviction should be reversed, but disagreed the law was unconstitutional.

A law on the books in the District of Columbia made it a felony to voluntarily be in a car where an illegal gun was present, even if it wasn't yours. Not anymore.

Conley's attorney, local solo practitioner Ian Williams, could not immediately be reached for comment. A spokesman for the U.S. attorney's office, William Miller, declined to comment. The Public Defender Services for the District of Columbia supported Conley's position on appeal; an attorney could not be reached.

Judge Stephen Glickman, writing for the majority, said the law was written in a way that required the defendant to prove he was in the car involuntarily, versus requiring the government to bear the burden of proving the defendant was in the car voluntarily. That type of burden-shifting was a violation of due process, the court found.

The majority found the law criminalized behavior no ordinary citizen would have reason to think was against the law. Citizens might know that guns are used to commit crimes, Glickman wrote, but that doesn't mean they're on notice "that the simple sight of a handgun triggers a novel legal duty to leave a motor vehicle at once, or to refrain from entering one."

The challenged law, Glickman wrote, "is an anomaly, a unique departure from the fundamental and intuitive premise of our legal system that one does nothing wrong and does not become a criminal merely by being a bystander to a crime." He noted that although ignorance of the law was not usually a defense against prosecution, Conley's case presented a rare exception.

Glickman was joined by Judge Kathryn Oberly. Judge Phyllis Thompson wrote a concurring opinion agreeing Conley's conviction should be reversed, but not because the law was unconstitutional. Thompson said the law required the government to prove the defendant knew the gun was illegal, and since the court didn't tell the jury about that standard, Conley's conviction should be vacated.

However, Thompson said the law did properly place the burden of proof on the government to show the defendant's presence in the car was voluntary, and then allowed the defendant to offer evidence he could not have reasonably left the car. Those respective burdens of proof were compatible and didn't unconstitutionally shift the burden to the defense, she wrote.

Thompson also disagreed that ordinary citizens would have no reason to think that being in a car with a gun might be criminal behavior. "Given the District of Columbia's longstanding law treating guns as dangerous weapons," she wrote, "I believe it is fair to say that the average member of our community who voluntarily and knowingly is in a vehicle with an illegal firearm 'knows that he is dealing with a dangerous device of a character that places him in responsible relation to a public danger,' and thus is 'alerted to the probability of strict regulation."