St. Paul, Minn. ? A labyrinth of caves left by 1800s sandstone miners along the Mississippi River has long been a forbidden and sometimes deadly thrill for teenagers, who ignore the keep-out signs and thwart the city’s efforts to seal the passages.

On Tuesday, the caves again proved lethal: Three teens died, apparently of carbon monoxide poisoning, perhaps from a fire smoldering inside the caverns.

Killed were Nicholas Lee Larson, Natalie Lorraine Vanvorst and Patrick Gerard Dague, all 17. Seventeen-year-old Justin Jensen was rescued, and his condition was upgraded Wednesday from critical to serious. A fourth boy, Jay Boucher, 19, escaped and alerted authorities.

Fire Chief Douglas Holton said the teens entered through a small opening, about 3 by 5 feet. Once inside, they could stand up, he said. The dead were found about 600 feet in.

At least five other people have died in the caves in the past two decades.

The passages stretch for miles along the river and are known as the Wabasha Street caves. A brewery once dug some of the caverns to create earthen warehouses, and a mushroom-growing operation flourished in the moist, dark caves for decades. The caves even had a nightclub in the 1930s, the Castle Royal, and mobsters were said to frequent the spot.

Over the years, the city has tried to keep people out by posting warning signs, boarding up openings with plywood, piling up sand in the entrances, and dumping thousands of tons of construction debris from razed buildings in the passages to fill them up.

But people shimmy through the openings or chip away at the soft sandstone at the blockades.

“You can just imagine, you close up some hole — cement it shut — then they just seem to dig around it,” Mayor Randy Kelly said.

A friend of Natalie Lorraine Vanvorst, 17, who died Tuesday in a series of caves along with Patrick Gerard Dague, 17, and Nicholas Lee Larson, 17, weeps while visiting the hillside memorial to the three teenagers in St. Paul, Minn.

Fire Chief Douglas Holton said two entrances that were sealed after a fire just two weeks ago were quickly pried open. He said it would be impossible to seal off the caves completely, because “there are entrances and exits that we don’t even know of.”

E. Calvin Alexander, a professor of geology and geophysics at the University of Minnesota, said the efforts to bar the caves might only have made them more dangerous: The wooden construction debris has provided fuel for fires, and closing up the openings has reduced ventilation.

The source of the carbon monoxide fumes was not immediately known.

Cave visitors sometimes start fires, creating a buildup of carbon monoxide in pockets in the caverns. But Holton said he did not believe the teens themselves started any fire Tuesday because there was no smoke in the cave and the group had flashlights.

Police said the teens had gone to the caves to explore after learning about them from friends. Boucher told police the group walked past a sign that warns that two other teens died in the caves in 1992.