A fishing boat being towed by a truck along a Staten Island boulevard came unhitched and struck two bicyclists, critically injuring one and killing the other, police said Tuesday. 

Alexa Cioffi, 21, was traveling with another female bicyclist on Hylan Boulevard near Redgrave Avenue at about 7:18 p.m. Monday when a Toyota truck towing the boat on a trailer in the northbound lane tried to switch lanes and overtake the bicyclists, police said. The boat came loose and and struck one bicyclist before hitting Cioffi and pinning her underneath as it came to a stop. 

Both bicyclists were taken to Staten Island University North Hospital, where Cioffi was pronounced dead, police said. The second bicyclist, who police have not named, was in critical condition, police said. 

One witness told the Staten Island Advance that she was in her home on Hylan Boulevard when she heard "two thumps." 

"I looked out the window from upstairs and I see the boat where it is, and I looked down and I saw a woman -- she's flailing around on the actual street," Nancy Maita told the newspaper. 

The driver stayed at the scene and no arrests were made, police said. The NYPD's Collision Investigation Squad is investigating.