CHICAGO (CBS) — Twice in nine hours on Tuesday, Chicago Fire Department divers were called upon to pull people out of the frigid waters of Lake Michigan, prompting officials to urge people to resist the urge to go for a swim.

Paramedics and doctors worked for two hours trying to save 13-year-old Tianna Hollinside, who was pronounced dead at St. Francis Hospital in Evanston after divers pulled her out of the lake near Rogers Park Beach.

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Hollinside was in the water for about 45 minutes before a diver found her.

Later Tuesday, around 7 p.m., the Fire Department got a call about two people in the water and in need of help at Montrose Beach. One of the swimmers was able to make it to the pier on his own, but the second was in the water for more than 30 minutes before divers pulled him out.

Juan Cornelio, 24, was taken to Weiss Memorial Hospital in critical condition, but was later pronounced dead, according to the Cook County Medical Examiner’s office.

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Fire Department spokesman Larry Langford said the water might look inviting, but it’s still dangerously cold.

“Be very careful out here. It looks like a wonderful summer day, but the water at 52 degrees is a very dangerous, dangerous item to deal with right now,” he said.

Langford said you can dip your toes in the lake, but otherwise sit out this week and wait until beaches officially open on Memorial Day, when the water should be warmer, and lifeguards will be on duty.

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“Sometimes people just go into shock, and they end up in terrible distress. So we’re encouraging people to stay out of the water right now. The beaches will be open in a couple of weeks with lifeguards, in a controlled situation, but right now the water is just way too dangerous to go into it. Again, if you’re experienced, it’s too dangerous. So people who are not experienced swimmers are in real trouble,” Langford said.