Jamming is one of the most severe attacks in wireless sensor networks (WSNs). While existing countermeasures mainly focus on designing new communication mechanisms to survive under jamming, a proactive solution is to first localize the jammer(s) and then take necessary actions. Unlike the existing work that focuses on localizing a single jammer in WSNs, this work solves a multi-jammer localization problem, where multiple jammers launch collaborative attacks. We develop two multi-jammer localization algorithms: a multi-cluster localization (M-cluster) algorithm and an X-rayed jammed-area localization (Xray) algorithm. Our extensive simulation results demonstrate that with one run of the algorithms, both M-cluster and X-ray are efficient in localizing multiple jammers in a wireless sensor network with small errors.