Cal-OSHA spokeswoman Patricia Ortiz said investigators were at the amusement park Sunday and are there Tuesday to make mechanical inspections.
There is no preliminary indication why the ride stopped around 3 p.m., Ortiz said.
Cal-OSHA engineers checked the ride before it opened in June and instructed Six Flags to train personnel and members of the Vallejo Fire Department on an emergency plan to how to evacuate the ride, Ortiz said.
The amusement park also was required to have a crane capable of reaching the top of the ride on site, Ortiz said.
The Vallejo Fire Department responded to the incident, and two members were raised 150 feet on a mobile platform to reach the roller coaster. The coaster was un-jammed and rolled backward to its starting point.
Ortiz said the park officials "acted efficiently."
Six Flags Discovery Kingdom spokeswoman Nancy Chan said the ride remains closed today "pending diagnostic and operational testing."
"Since the ride stoppage occurred on July 29, the ride has run efficiently and with no issues throughout the testing cycles," Chan said.
Ortiz said there is no deadline for Cal-OSHA to finish its investigation. Cal-OSHA has six months to complete any investigation of an incident that caused a loss of life or injury, she said.
This is not the first time a ride at Six Flags in Vallejo has malfunctioned. In August 2010, a problem on the Boomerang Coast to Coaster left 26 passengers stranded for an hour.