A TOW Missile knocked down the Russian attack helicopter in Syria?

via Japan Times

Two Russian airmen killed in Syria on Friday were shot down with American weaponry, the Interfax news agency said Sunday, quoting a Russian military source.
It said insurgents from the Islamic State group hit the airmen’s Mi-25 assault helicopter with a U.S.-made TOW heavy anti-tank missile, a weapon that uses guidance from a ground station.
The high-tech system is in service with the U.S. military.
“According to information we have, the terrorists used an American TOW,” Interfax quoted the unnamed source as saying.
The Islamic State group on Saturday claimed responsibility.
Amaq, a news agency that disseminates the group’s propaganda, posted online video purporting to show the incident.
The video shows a flash of light at the helicopter’s tail, and the rear rotor unit appears to fall away. The helicopter then goes into a spin before crashing, to cries of “God is greatest” in audio accompanying the images.
As the aircraft goes down, a second helicopter passes by. It is of similar appearance to the stricken Mi-25.

A wire guided anti-tank missile is capable of knocking down a modern Russian attack helicopter?

Quite honestly I didn't think it possible.  Is this a case of tactics?  I know that the Army Apache drivers like the "hover then pop up and shoot" whereas Marine Corps Zulu jocks drive'em like they stole'em.  Its all gun and missiles runs in swooping attacks from various directions.

Has the engagement envelope for the TOW expanded and we missed it or is it a case of tactics making the Russians vulnerable.