Communication with Mitsubishi FX3G over USB Cable

We have a situation where we need to communicate with a Mitsubishi FX3G PLC, but the problem is that only the USB port of the PLC is available (buying new Mitsubishi modules is out of scope). I couldn't find which protocols are available at the USB port, neither which protocol the GX Software uses to monitor the PLC I/Os states over USB.
Does anyone knows how we can use the USB port to collect data from this PLC?