Carbon monoxide poisoning is suspected of killing a 24-year-old woman on a boat over the weekend on the St. Croix River near Bayport, authorities said Monday.

Two men also on the pleasure craft were hospitalized Sunday after they fell ill, according to the St. Croix County Sheriff's Office.

The three had plans to camp at a beach starting Saturday night, but no one had been able to reach them later, the Sheriff's Office said.

The woman was identified as Ashley G. Speer, of Glenwood City, Wis. Speer was pronounced dead at the scene.

Justin M. Roskos, 25, of Hastings, and Hayden L. Johnson, 27, of River Falls, Wis., are being treated at Hennepin County Medical Center. Roskos, the boat's owner, is in critical condition, while Johnson is in satisfactory condition, a hospital spokeswoman said Monday afternoon.

"Initial investigation indicates this is a probable carbon monoxide poisoning incident," a statement from the Sheriff's Office read. "However, the investigation is ongoing."

Sheriff's Capt. Jeff Klatt said all three were found unconscious in the boat's cabin.

Carbon monoxide can build up from an idling motor, generator or faulty motor exhaust system. The poisonous gas is odorless and invisible.

Klatt said the boat was equipped with a generator, but "we're not sure whether it was running at the time" when authorities located the boat.

Wisconsin authorities received requests for a welfare check late Sunday afternoon on the three and their boat, a 34-foot Wellcraft. About 15 minutes later, the boat and its occupants were located by the Washington County water patrol at Hi-Line Beach, which is on the Wisconsin side of the river across from the Xcel Energy plant in Bayport.

The boat had a carbon monoxide detector, but "we're checking now to see whether it was operational," Klatt said.

Minnesota became the first state in the nation to require carbon monoxide detectors in some boats. The law takes effect in May 2018. In the meantime, conservation officers and others in law enforcement are educating boaters about the law and what will be required.

The statute is called Sophia's Law, named for Edina 7-year-old Sophia Baechler, who died in 2015 on Lake Minnetonka when carbon monoxide leaked from a boat's exhaust pipe.