Complex sentence: The discovery of the bodies created a greater sense of urgency for Brocchini and Buehler, who knew that Peterson was in San Diego at the time, because they had put a tracker on his car, and who feared he would escape across the border to Mexico.
Split into simpler sentences: The discovery of the bodies created a greater sense of urgency for Brocchini and Buehler, who had put a tracker on Peterson's car. Knowing that he was in San Diego at the time, they feared he would escape across the border to Mexico.