Emergency crews responded to Porter Beach about 6:50 p.m. after it was reported the two needed to be rescued while swimming, Porter police said.
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A Good Samaritan was able to pull the girl to shore, but the man could not be found for about 20 minutes, police said. When he was pulled out of the water, first responders performed life-saving measures.
The man was taken to Northwest Health, where he died, police said just after 9 p.m.
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Family identified him as Pedro, but did not want to share their last name. The 14-year-old girl who was rescued is his cousin Anna. She said they were hit by a big wave, and her cousin couldn't swim.
Anna was taken to a nearby hospital where she is reportedly, "doing fine," police said.
A spokesman for the Great Lakes Surf Rescue Project said a wind shift from the north created waves for the south prior to the incident.
Porter police and Indiana State Police Conservation Officers are investigating.