But after several minutes of performing CPR to no avail, he realized the baby was just a doll.
"All indications were that it was a real baby," Lieutenant Shane C. Maxfield from the Keene Police Department told InsideEdition.com.
According to Maxfield, Lieutenant Jason Short was dispatched after three different bystanders reported seeing a baby left in the back of a car on a day when temperatures were exceeding 90 degrees.
When Short arrived on the scene, he said he saw a car seat with a blanket draped over most of it. Peeking out from underneath the blanket, however, were two tiny legs.
Within seconds, Short made the decision to shatter the window with his baton.
"With infants and heat, seconds or minutes are vital so he made the decision immediately to break a window and get him," Maxfield said.
Even when Short, who is a father himself, grabbed the baby from the car seat, he said the arms appeared to spring up as if it were alive.
"Its eyes were closed, looked puffy and the skin was kind of hot and dry, like someone with heatstroke might have," Maxfield said. "He started performing little baby CPR."
Several seconds later, the lieutenant noticed he wasn't able to get any air in. Then, he noticed it that it was not even a real baby.
"It was very upsetting during the time," Maxfield said. "It's certainly traumatic dealing with dead or injured children."
According to Maxfield, officers soon discovered the doll, named Ainslie, was known as a reborn doll, which has been created to look extremely lifelike.
Their "skin" is meant to mimic that of a human, and they are even as heavy and dense as a real infant. While most reborn dolls cost approximately $200, Maxfield said Ainslie cost $2,000.
"With that expense goes quality of construction," Maxfield told InsideEdition.com. "It was absolutely convincing."
Ainslie is one of 40 dolls in her owner's collection.
According to her owner, the dolls help her deal with her own son's passing from Hunter's disease only a year and a half ago.
"I am under a huge amount of stress, and I do my best to cope," she told InsideEdition.com. "I was excited to have this type of doll and wanted her with me."
When she finally returned to her vehicle, officers said she was quite upset, but "eventually she understood why people would react. She thought she had the whole thing covered up."
After a lengthy conversation between the woman and the police department, she bought a bumper sticker for her car that explained the doll is not a real baby to prevent the situation from happening again.
The police department has also reportedly agreed to pay for the shattered window.