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We present a search for a narrow-width heavy resonance decaying into top quark pairs (X →tt) in p p collisions at √s = 1.96 TeV using approximately 0.9 fb−1 of data collected with the DØ detector at the Fermilab Tevatron Collider. This analysis considers tt candidate events in the lepton plus jets channel with at least one identified b jet and uses the tt invariant mass distribution to search for evidence of resonant production. We find no evidence for a narrow resonance X decaying to tt. Therefore, we set upper limits on σX • B (X →tt) for different hypothesized resonance masses using a Bayesian approach. For a Topcolor-assisted technicolor model, the existence of a leptophobic Z′ boson with mass MZ′ < 700 GeV and width ΓZ′ = 0.012 MZ′ can be excluded at the 95% C.L.