Judith Fleissig, 58, of Rochester, New York, rented the Nissan Rogue 4x4 and took it on a driving holiday in Florida when her daughter noticed a strange bag in the back of the vehicle.
> Opening it up expecting to find rental documents and other paraphernalia, she instead found the lethal AR-15 assault rifle, which is capable of firing up to 800 rounds per minute in fully automatic versions.
"We got out of the car, we were kind of freaked out," she told Florida newspaper the Sun Sentinel. "I didn't want to touch it."
The weapon had been left in the car by the previous hirer, model Lauren Tannehill, wife of Miami Dolphins quarterback Ryan Tannehill.
She had only been in the car a few hours, before returning it in exchange for another, but had forgotten to remove her rifle, which was found complete with a military-style scope and bipod.
Ms Tannehill will not face any charges over the matter as she legally owns the gun. However, she did have to wait to collect it from police, who ran checks to find out whether it had been stolen.
"Though dangerous if it would have landed in the wrong hands, it's not criminal," Broward sheriff's spokeswoman Keyla Concepción told the Sun Sentinel. "They simply forgot it."
A spokesman for E-Z Rent-A-Car told the newspaper that their cars are cleaned when returned by customers and that he was "not sure" how they missed the weapon during checks.