We search for resonant production of tt pairs in 4.8  fb−1 integrated luminosity of pp collision data at s√=1.96  TeV in the lepton+jets decay channel, where one top quark decays leptonically and the other hadronically. A matrix-element reconstruction technique is used; for each event a probability density function of the tt candidate invariant mass is sampled. These probability density functions are used to construct a likelihood function, whereby the cross section for resonant tt production is estimated, given a hypothetical resonance mass and width. The data indicate no evidence of resonant production of tt pairs. A benchmark model of leptophobic Z′→tt is excluded with mZ′<900  GeV/c2 at 95% confidence level.