The weapon was recovered in a bus operating between Kashmere Gate ISBT and the JNU campus, after the driver spotted an unclaimed bag and raised an alarm.
Police is trying to ascertain identity of the bag's owner and several persons have been questioned, a senior official said on Friday.
With the gun, there was also a letter which said that Kanhaiya and Umar Khalid, who were arrested under sedition charge over a controversial event in JNU campus, will be decapitated.
The letter was purportedly written by the person who had threatened Kumar over Facebook earlier saying that men with weapons are already present inside the campus, ready to kill him any moment, the official said.
Kumar doesn't get security cover inside the campus but the university authorities have clearly been instructed to inform Vasant Kunj (North) police station everytime he leaves the campus and security is provided accordingly.
After the alert, the police are likely to increase his security cover and also enhance vigil around the JNU area, the official added.
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