Where in powerful jets of distant active galaxies – the mightiest and most energetic objects known – are the violent outbursts of high energy gamma-ray emission produced? Very close to the central supermassive black hole and accretion disk powering these systems, or at larger distances from the "central engine", i.e. further downstream in the jet? New insights into this long-standing question became possible recently, thanks to intensive, multi-frequency radio observations of powerful active galaxies.