Suspicion of foreign espionage, cursive messages in ancient Chinese, a sensitive microchip — and a suspect that could not be stopped at the border.

Ravindar Patil, the assistant Mumbai police sub-inspector assigned to the case, was scratching his head for answers. But first, he had to find a place to lock up the unusual captive.

So he turned to a veterinary hospital in the Indian metropolis, asking it to retrieve a list of “very confidential and necessary” information about the suspect — a black pigeon caught lurking at a port where international vessels dock.

“The police never came to check the pigeon,” said Dr. Mayur Dangar, the manager of the hospital.

After eight months, the bird was finally set free this week, its innocence of spying for China long confirmed through crack detective work, but the jail doors flung open only after a newspaper report, repeated letters to the police by the veterinary hospital, and intervention from an animal rights group.

The group, PETA India, celebrated what it called the end of a “wrongful imprisonment.”

“PETA India handles 1,000 calls a week of animal emergencies, but this was our first case of a suspected spy who needed to be freed,” said Meet Ashar, who leads the organization’s cruelty response division.

Mr. Ashar said the case had put the hospital’s staff members in a dilemma: They didn’t want to expose a healthy bird to the sick and injured, but they also couldn’t set it free because “it was such a high-profile case and the charge was so serious.”