It’s not just the amount of firepower now in separatists’ hands that is alarming observers. It’s also the kind of weaponry—specifically, the Strela-10 anti-aircraft missile system.

The Strela-10 is designed to supply close support to troops and vehicles near the front line of fighting. It’s meant to shoot down fast-moving, low-flying aircraft such as helicopters or non-stealth jets. There is significant evidence that the Russian military regularly includes the Strela-10 in important convoys because it can protect against immediate air threats while longer-range weapons (such as the Buk system, which shot down a MH17 passenger jet in 2014) can perform the same function from further afield. In fact, the first time the Russian-backed fighters were ever seen using T-72 tanks, Strela-10s were filmed escorting the vehicles from a border crossing in Lugansk just seven days before the MH17 was immolated. There are no documented cases of the Strela-10 system having been captured by Russian-backed separatists, and the T-72 has not been used by the Ukrainian military in this conflict.

On May 2, three Strela-10s were spotted escorting a convoy of two T-64B tanks, three BMP-2s, three BTR-80s, three 2S1 Gvozdika self-propelled howitzers, three BM-21 Grad MLRS, three trucks towing artillery pieces, perhaps 2A65 152mm howitzers, and several other military trucks and fuel tankers through Lugansk. On May 5 Strelas were seen parading through Donetsk, escorting a similar convoy. But if they were only in Donetsk for the May 9 parade, it’s curious that a similar convoy was spotted by the OSCE within the line of demarcation near Donetsk on May 10, suggesting that the weapons have not returned to their holding areas as specified in the Minsk protocol.