“It was informed by the handler that opposite to DRM office, there are two trees and on one of the trees there would be a white color polythene sheet (used for wrapping fragile articles),” Mr. Mohammed told investigators. “We spotted the place, and I, first on the pretext of urinating, went to check for the consignment.”

When they opened the bag, they found two pistols and 20 bullets, according to their account to law enforcement. It was one of at least four drops that the handler set up for them.

Because the pistols were rusted, they say the handler instructed them to travel to the railway station in the city of Ajmer, about 600 miles to the north. This time they were told to bring 65,000 rupees — around $1,000 — and leave it near the railway track sealed in a plastic bag, which would be picked up and used as payment for weapons.

Because the communication always had to go through the handler, the members of the Hyderabad cell never directly interacted with the arms seller. When they were arrested, they could not provide any clues as to who had left the contraband, Indian investigators said.

The Hindi-speaking handler guiding the men in Hyderabad also insisted on using a kaleidoscope of encrypted messaging applications, with Mr. Yazdani instructed to hop between apps so that even if one message history was discovered and cracked, it would reveal only a portion of their handiwork.

As soon as Mr. Yazdani indicated he was willing to undertake an attack, the handler instructed him to download ChatSecure, a messaging app to be used when they spoke by phone. When he used his laptop, he was told to contact the handler via Pidgin, another encrypted tool. He was told to create an account with Tutanota, a secure email service. And the handler taught Mr. Yazdani how to use the Tails operating system, which is contained on a USB stick and allows a user to boot up a computer from the external device and use it without leaving a trace on the hard drive.

Once that system was in place, the handler told Mr. Yazdani to prepare a handwritten oath of allegiance, known as a “bayah,” to the Islamic State’s leader.