A lioness killed a forest guard while a lion escaped from its enclosure in the Sanjay Gandhi National Park on Tuesday evening, officials said.
The attack took place due to the alleged negligence of the forest guard as he left the door of the enclosure open during the evening feeding time of the lions, an official said.

According to an official, Harichand Gangwan, the forest guard, had gone to feed the five lions and lionesses in the enclosure at around 6 p.m. but he apparently forgot to shut the trap door separating the wild animals and the feeding area.

Seizing the opportunity, one of the two lionesses rushed in and pounced on Gangwan, even as a couple of other forest guards watched in horror.

Regaining their composure, one of the guards tried to shut the trap door to prevent the other three lions from escaping. But one of them proved too fast and bolted out.

Additional armed forest guards were summoned from the national park headquarters located around two kilometres away and they tried to chase the 'killer' lioness away and collect Gangwan's body.

But the lioness, growling ferociously, refused to let go of the body for nearly an hour. The forest guards later shot a tranquilizer and immobilised her.
However, till late Tuesday they could not retrieve Gangwan's body since one lion was still at large in the national park, the official said.

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Source : freepressjournal

Please explain to me the emboldened parts.

Though I guess "go of the body" means "go away from the body", and "shot a tranquilizer" means "shot a tranquilizer (a drug bullet) from a gun".

You are looking at it the wrong way. Rather than "go of the body," you should focus on "let go of the body." Let go is a phrasal verb meaning "to release" or "to relinquish" something.

A tranquilizer is an agent or drug used to calm or sedate a living being, human or animal. In this context it means that they shot her with some form of dart containing a powerful sedative.

At large is an idiom meaning—in this context—"free," "at liberty," or "not in confinement." In other words, they have still to capture the last lion.
Thank you Falconer.

I know "let go" means release but I wonder what "let go of" means here. If let go means "release", the sentence becomes "release of the body" which doesn't make much sense to me. It should have been "release the body", what the "of" means here.