Testing the event horizon paradigm with VLBI probes of magnetic field

Arguments put forth so far for the existence of cosmic black holes have been largely based on various types of enclosed mass measurements and observations of relativistic effects arising in matter and photons moving near the event horizon. The GRAVITY observations of stellar motions near Sgr A* and the EHT imaging of M87 and Sgr A* illustrate well the success of these approaches for testing the black hole and event horizon paradigm. Further effective tests of the event horizon paradigm can be carried out with VLBI probes of magnetic fields around black holes on scales below several thousands gravitational radii. This possibility is recognized and explored by our project M2FINDERS focusing on multifrequency polarization VLBI imaging of compact radio emission in selected AGN in which linear scales below about 10000 gravitational radii can be probed. These imaging yields a combined information on linear polarization, Faraday rotation, and synchrotron spectrum and opacity, which provides a robust framework for estimating the strength and reconstructing the three-dimensional morphology of the magnetic field in the vicinity of putative supermassive black holes in AGN. Current progress and first results from the M2FINDERS measurements will be presented in this contribution