This paper investigates the problem of line outage detection in an area external to the operating area boundaries. The so-called “internal system” is assumed to be fully observable and the base case topology of the external system is assumed to be known. A limited number of phasor measurements are also assumed to be available from the external system. The paper takes advantage of the equivalent representation of a line outage by a pair of injections at the line's terminal buses and formulates the outage detection problem as an integer programming problem. Results are verified by simulating various external line outage scenarios using the IEEE 118-bus test system.