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A population of intermediate black holes openParIMBHclosePar will allow us to solve one of the long-standing problems in modern astrophysics comma the origin of supermassive black holes. Having analysed 1 million optical spectra comma we identified 305 IMBH candidates openPar3e4-2e5 MSunclosePar comma which reside in galaxy centers and pose as active galactic nuclei. With Chandra DDT and XMM-Newton AO-18 observations and mining X-ray data archives comma we confirmed the AGN nature of 11 candidates comma thus validating our IMBH search approach. Here we propose to observe 12 additional optically selected IMBH candidates to double the number of X-ray confirmed accreting IMBHs. This will enable detailed studies of the IMBH population and help to constrain the masses of ..seed. black holes in the early Universe.