I will present a search for heavy pseudoscalar or scalar bosons decaying to a top quark pair (tt̅) in final states with one or two charged leptons, using 138/fb of proton-proton collisions at √s = 13 TeV recorded by the CMS experiment. The invariant mass of the reconstructed $t\bar{t}$ system and variables sensitive to its spin state are used to discriminate against the standard model tt̅ background and to infer spin quantum numbers. An excess of the data above the background prediction, as modeled using perturbative quantum chromodynamics (QCD) only, is observed with a significance of above five standard deviations. I will discuss three possible hypotheses to interpret the excess, which is located close to the tt̅ production threshold: by production of an additional scalar or pseudoscalar boson, or by the existence of a color singlet pseudoscalar tt̅ bound state, as predicted in a simplified model of nonrelativistic QCD. For the first two hypotheses, I will outline upper limits on the coupling of pseudoscalar or scalar bosons to top quarks in a mass range of 365–1000 GeV and relative widths of 0.5–25%. For the third scenario, I will present and discuss an extracted cross section of 7.1 pb with an uncertainty of 11%.