MILWAUKEE — A recent fad in which people challenge their friends to jump into freezing water to raise money for charity has prompted warnings across the nation, as an increasing number of divers are reporting broken bones, blown-out knees or worse.

The phenomenon, which began last month as a fundraiser for a 6-month-old cancer patient, has evolved as it spread. In some cases those who are challenged must pay $100 if they fail to take the plunge within 24 or 48 hours. In other cases, people jump and still donate to charity. In another variation, the challenge is a straightforward dare with no charity involved.

Although the challenges are well-intentioned, injuries have mounted in states including Wisconsin, Illinois, Iowa and Minnesota. Officials with high schools and state agencies are now urging people to forgo a jump or at least take precautions.

A 16-year-old girl in Fond du Lac who recently jumped into Wisconsin’s Lake Winnebago in a challenge from a friend shredded ligaments in her knee when she landed on razor-sharp shells. Elsewhere, officials at New Trier High School in Winnetka, Ill., alerted parents to the practice after one student fractured an ankle jumping into Lake Michigan and another suffered a head injury that required stitches.

The idea isn’t new. The Special Olympics has polar plunges in which volunteers get friends to sponsor their jumps into freezing water. Emergency crews are always on hand for those plunges.

The cold-water challenge trend took off after the family of Landon Shaw — an infant from Tarkio, Mo., who has a rare form of cancer — created the “Plunge for Landon” challenge, which raised more than $250,000. There was at least one severe injury: An Iowa man broke two vertebrae when he dove into water that was shallower than he realized.

Landon’s mother, Alyssa Shaw, said her family donated money to the man, and she says anyone thinking about jumping into cold water should put safety first.

Officials in Wisconsin and Minnesota urge those who take a plunge to never go alone, wear a life jacket and dry off quickly.

Despite problems, several charities have benefited from the jumps. The Steuben County Cancer Association in Angola, Ind., raised about $1,500 since one local sheriff’s office challenged another to take the plunge last month.

In that case, the divers agreed to jump and also donate money, said Candy Oliver, the cancer association’s executive director.

“I did it. I’m 60 years old, the ice had just come off the lake and when I went under I thought I was going to have a heart attack,” she recalled. “It was horrible, but it was for a good cause. The thing is, if it raises money for someone who doesn’t have money it’s worth every minute.”