Abstract : A model-independent search is presented for the production of heavy resonances decaying into top-antitop quark pairs, seen as excesses above the standard model prediction in the t t-bar invariant mass spectrum. Events containing one lepton (muon or electron) and at least two jets are selected from data samples corresponding to 4.4-5.0 inverse femtobarns of integrated luminosity collected by the CMS experiment in pp collisions at sqrt(s) = 7 TeV at the LHC. Results are presented from the combination of two dedicated searches optimized for boosted production and production at threshold. No excess of events is observed over the expected yield from the standard model processes. Topcolor Z' bosons with narrow (wide) width are excluded at 95% confidence level for masses below 1.49 (2.04) TeV and an upper limit of 0.3 (1.3) pb or lower is set on the production cross section times branching fraction for resonance masses above 1 TeV. Kaluza-Klein excitations of a gluon with masses below 1.82 TeV (at 95% confidence level) in the Randall-Sundrum model are also excluded, and an upper limit of 0.7 pb or lower is set on the production cross section times branching fraction for resonance masses above 1 TeV.