The shootings appear to have occurred after robberies or attempted robberies at the four convenience stores on July 11, or 7/11 - a day when the national 7-Eleven brand is celebrating its 95th birthday by giving out free Slurpee drinks.

'Our hearts are with the victims and their loved ones,' 7-Eleven, Inc. said in a statement. 'We are gathering information on this terrible tragedy and working with local law enforcement.'

It wasn't immediately clear to investigators what prompted the shootings in the cities of Riverside, Santa Ana, Brea and La Habra, or why the violence occurred on July 11.

'I think the only person to answer that would be the suspect,' said Officer Ryan Railsback, a spokesperson for the Riverside Police Department, where the first shooting happened at about 1:50 a.m. 'There´s no way it can be a coincidence of it being 7-Eleven, July 11.'

The Riverside shooting left the victim in grave condition and while that shooting had not been officially connected to the others, Railsback said, 'they all seem very, very similar.'

The cases appeared strange, Railsback said, because criminals typically know that robberies at convenience stores rarely yield large amounts of money, especially during the overnight hours.

'If you go hit a liquor store or a 7-Eleven or a fast food place, you´re not going to get a lot of cash out of it,' he said. 'It´s kind of odd that they would do this.'

In the Riverside case, the gunman robbed the clerk and brandished a gun, then turned the weapon on a customer, opened fire and fled, Railsback said. Police believe the clerk handed over cash from the register.

'It doesn´t appear to be any reason that the suspect shot the customer,' Railsback said. 'It sounds like the clerk gave him whatever he asked for.'

The second shooting occurred around 3:20 a.m., about 24 miles (39 kilometers) away, in Santa Ana, authorities said.

Officers responding to reported gunfire at a 7-Eleven in that city and found a man dead in the parking lot with one gunshot wound in his upper torso, according to Sgt. Maria Lopez, a Santa Ana police spokesperson. The violence appeared to include a robbery.