"We know people saw something," sheriff's spokesman Steve Whitmore said. "And we need them to come forward and help us solve this crime."
San Gabriel police Sgt. Jeff Whitney said several gangs operate in the city, though he did not know which one might have been involved in the shooting.
The Coffee House is located in a strip mall along busy Valley Boulevard. A Chinese bakery, foot massage parlor and Vietnamese restaurant are nearby.
Four people who appeared to work at the cafe declined to speak to a reporter as they mopped up pools of blood and swept away blood-soaked playing cards and glass from a smashed table top.
Signs of the killing were gone by midday, and the cafe was open, serving coffee and other hot drinks to about 10 men.
Owners and customers at surrounding businesses were reluctant to discuss the incident. Most said they did not speak English.
One person, who asked to remain anonymous for fear of retaliation, said the shop was a popular nighttime hangout for young people, and police had been called there in the past to break up fights.
The shop's surveillance cameras were not working properly and did not record the attack, San Gabriel police Sgt. Rene Hernandez said.
Responding officers who received a panic alarm call from the shop were flagged down by several people who had been shot, he said.