1. A method for distributed tracking of multiple targets using a variational Expectation-Maximization (EM) algorithm in a surveillance system, the surveillance system comprising a plurality of trackers tracking the multiple targets, the method comprising: estimating a motion state variable of each of the plurality of trackers, the estimation being performed in the E-step of the variational EM algorithm; estimating a data association variable of each of the plurality of trackers based on a graph-based discrete optimization algorithm, the estimation being performed in the M-step of the variational EM algorithm; tracking the multiple targets based on the motion state variable and the data association variable of each of the plurality of trackers; and wherein the data association variable of each of the plurality of trackers is estimated as a maximum a posteriori (MAP) estimate in the M-step of the variational EM algorithm.