An alligator has taken up residence in a Chicago lagoon, surprising locals after a winter of polar temperatures in the third-largest U.S. city.

Police confirmed the reptile had been spotted on Tuesday in Humboldt Park Lagoon, on Chicago’s west side. It was still eluding capture on Wednesday despite pledges from local officials to trap it.

Authorities aim to have the animal, estimated to be 1.2 to 1.5 metres long, humanely trapped and relocated to a zoo for veterinary evaluation, said Chicago police spokesperson Anthony Guglielmi on Twitter.

Temperatures in Chicago on Wednesday were hovering above 30 C, the range in which alligators are most active, according to the Florida Fish and Wildlife Conservation Commission.

Alligators can go up to two months without eating, and capturing the critter could be time consuming, said Rich Crowley, president of the Chicago Herpetological Society, a non-profit organization dedicated to reptiles and amphibians.

“The alligator is still very shy,” said Crowley, explaining it was likely once a pet and was still getting used to its new surroundings. Crowley estimates the alligator to be likely five to 10 years old. They can live to be 80.

Alligator Bob to the rescue

Officials warned residents against attempting to capture the alligator on their own and said they had sent out an expert known as Alligator Bob to snag the gator.

Alligator Bob is a volunteer who declines to have his full name used. He works with Chicago officials to capture exotic animals, according to Crowley.

Alligator Bob, an animal expert with the Chicago Herpetological Society, sets alligator traps in Humboldt Park Lagoon on Wednesday. (Ashlee Rezin/Chicago Sun-Times/The Associated Press)

“Alligator Bob is the hero we all deserve right now. Give him space and let him humanely catch our new friend,” Chicago’s 14th District Police Department said on Wednesday in response to several offers of help on Twitter.

American alligators normally live in freshwater wetlands and marshes in the southeastern United States.

The police department warned residents against keeping baby reptiles, pointing out that they could pose a danger once they grow up.