Michael Mohel, deputy chief of detectives for the Ocean County Prosecutor's Office, said a dispute between brothers Bryan and Craig Mueller prompted Bryan Mueller to move out of their home Monday morning.
Bryan Mueller was loading his possessions into a vehicle and was about to leave when his brother opened fire with a high-powered rifle, killing him and Cara Ellis, a neighbor who had come to his aid.
It was not immediately clear what the brothers were arguing about.
"Our investigation determined that Bryan was in the process of moving out," Mohel said. "He was observed moving items out of the unit. We received information that he was leaving the premises that day. Whether it was to be permanent or not is still to be determined."
Authorities said Craig Mueller opened fire on his brother from a second-floor window Monday morning, spraying the area with gunfire that also claimed the life of Ellis, a neighbor who was walking by and ran to Bryan Mueller's aid.
Craig Mueller later turned the gun on himself.
The shooting forced evacuations of neighbors in the Tall Timbers housing development some 35 miles northeast of Atlantic City and prompted a lockdown at township schools until mid-afternoon because police believed the gunman might have been on the loose.