[en] The results of a measurement of the orthopositronium (o-Ps) decay rate, λ_T, using the vacuum technique are presented. This 230-ppm measurement is consistent with a recent 200-ppm measurement of λ_T in gases and disagrees with theory at the 6.2 sigma level. Experimental searches for alternative o-Ps decay modes that might resolve the discrepancy are reported. A completed search for two-photon decay modes, both QED-allowed and forbidden, shows that they cannot be responsible for the discrepancy at the 233-ppm level. (author) 18 refs.; 4 figs