The Tor system is a mobile missile launch system, with eight missiles carried on either a tracked vehicle or a truck. The vehicles can operate without relying on other air defense infrastructure.

They carry both a radar to detect targets and a launch system. The low- to medium-altitude missiles were developed by Soviet engineers in the 1970s as a so-called lower-tier air defense weapon.

Russia sold the Tor systems to Iran in 2005 as part of a $1 billion arms deal and over the objection of American diplomats. It has also sold the system to more than a dozen other countries.

A New York Times analysis of flight path information and video of the missile strike determined that the plane stopped transmitting its signal for between 20 seconds and 30 seconds before it was hit.

Civilian airplanes identify themselves with radio signals constantly streaming from a system known as a transponder on the planes, said Ian Petchenik, a spokesman for Flightradar 24, which tracks the signals for flights around the world.

The Tor software relied on radar and visual identification of a plane as well as the identification signals from the transponder, John Cox, an accident investigator and former pilot who is the chief executive of Safety Operating Systems, said. If the identification is incorrect or absent from the plane, Mr. Cox said, the system “will declare it a threat.”

From there, he said, the missile navigates via radar, “and when it gets in proximity to target it explodes,” releasing deadly fragments. A second missile is usually fired immediately after the first.