A Texas inmate was executed on Thursday for the brutal murder of a woman in 2002, the motive for which appeared to be burglary. However, prosecutors say they believe the man was on his way to many, many more killings before he was caught.

“A girlfriend told us he kept a list in notebooks of names and license plates of girls he would follow,” prosecutor Lance Larison said. “I think he was working up to something.”

The man, 33-year old Mario Swain, broke into 46-year old Lola Nixon’s home just after Christmas in 2002 and beat, strangled, and stabbed her to death before dumping her body in an abandoned car several miles away. Evidence in the home showed obvious signs of a struggle, and witnesses–along with a trail of physical evidence–led police to Swain. Once he was arrested, officials discovered his M.O.: either drugging women, beating them over the head to knock them out, or using a stun-gun on them before robbing them.

Swain’s execution marks the 13th for the state of Texas this year, more than any other state in the nation.