In situ electric field (E) measurements and inferred lightning initiation locations of three cloud-to-ground flashes are used to identify a thunderstorm region in which the preflash E exceeded the threshold for runaway breakdown. The maximum measured E in the region was 186 kV m-1 at 5.77 km altitude, which for runaway electrons is equivalent to 370 kV m-1 at sea level; this E value is ~130% of the estimated threshold for an avalanche of runaway electrons. In addition, the volume where E exceeded the runaway threshold was estimated to be 1--4 km3, with a vertical depth of about 1000 m. At least within part of this volume the characteristic scale height for exponential growth of runaway electrons was 100 m or less. Thus for these three flashes the electric field conditions necessary for runaway breakdown existed, and runaway breakdown could have initiated the flashes.