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.”

It is not the first time that India has feared feathered infiltration, but the latest case was a sign of changing times and threats.

In 2014, the authorities in the Himalayan region of Kashmir, at the center of tense relations between India and Pakistan, arrested a pigeon near the border on similar charges.

The bird in Mumbai suggested new twists — it had appeared in a city nowhere near a contested border, and the Chinese writing inked on its wings pointed to a more sophisticated and powerful rival that India has been grappling with in recent years.

Mr. Patil, the 39-year-old sub-inspector, had dealt with two animal cases before in his 12-year career: the death of two dogs, one in a suspected poisoning that required a postmortem, and the other in a road accident. Neither case had geopolitical ramifications.

This time, however, “I had to ask advice from our intelligence colleagues,” he said in a phone interview.

The bird had been spotted by guards with the Central Industrial Security Force, which watches over government facilities like ports. Not the first to cast a critical eye on a pigeon, the duty officer saw this one loitering alone — “it was just sitting there, and it all looked suspicious to them — chip, and ring on the feet,” Mr. Patil said. The guards informed the police.

Once Mr. Patil found a place to lock up the bird, the slow work of investigation began. And he started piecing together clues.

The rings on the bird’s legs, including one that had a chip, were sent to the forensic sciences lab.

“The chip had details of the location coding — what it is, where it has come from,” he said.

“Nothing else turned out suspicious,” he added.

He cross-checked the details with information online and concluded that the pigeon was a racing bird from Taiwan. In speaking to the guards at the port, which mostly receives oil vessels bringing crude for refining, he learned that Taiwanese ships were among those that docked there. He deduced that the bird had probably reached Mumbai on one of the ships.

“It may have been weak and injured, and boarded the ship and off-boarded here,” he said.

As for the cursive Chinese writing on the wings?

“It was not readable,” he said. “Because it came by sea, it may have faded.”

Just why the bird remained in lockup for several months after Mr. Patil had completed his investigation is a matter of disagreement. The hospital and PETA say the police were not responsive and had essentially forgotten about the bird. Mr. Patil said the hospital had misread instructions that the pigeon should be freed once in good enough health.

The pigeon “looked no different from our pigeons,” Dr. Dangar said, and had done well on a local diet of wheat, millet and rice. So after the police finally responded to inquiries from the hospital and PETA with a “no objection” letter, it was set free on Tuesday.

Asked what he would say if the pigeon’s Taiwanese owners came to claim it, Mr. Patil said the bird had a new home in Indian skies.

“Now it belongs to us, here,” Mr. Patil said.