Officer Ariel Acosta saves Sean Mulligan

Officer Ariel Acosta is interviewed at police headquarters.

Geraldine Allen on Thursday had a few of her 14 grandchildren in her care when a metal window blind chain got wrapped around her 3-year-old grandchild Sean Mulligan’s neck.

Prince George’s County Police officer Ariel Acosta happened to be working a traffic stop blocks away. He arrived a minute later and instructed Allen to assist with CPR.

It was the first time Acosta had ever done it outside of that crucial training.

“He was not breathing,” Acosta said. “The family was in shock.”

But then Sean let out of breath of air. And they knew he was getting air into his lungs.

"It was a miracle, like a blessing from God, when he started crying is was such a good sound, and we all felt relief,"Acosta said.

Sean was airlifted to Children's Medical Center, where he remains sedated to let his body heal.

The chain that nearly cost him his life was cut down. It’s something the grandmother begs all families to do.

"Take them down,” Allen says.