Two women reportedly suffered life-threatening brain injuries after riding the X2 roller coaster at Six Flags Magic Mountain in California within six days of each other in July. Both required emergency brain surgery, and doctors cited rapid acceleration and deceleration on the ride as the likely cause; the incidents raise concerns about the safety oversight of one of the world’s most intense roller coasters.

Pamela Guillen, 40, reportedly lost consciousness after repeatedly hitting her head against the headrest, underwent surgery to relieve pressure on her brain and is now relearning aspects of daily life. Naomi Greer-Wilkinson, 25, collapsed after the ride, remains in a coma on a ventilator, and underwent emergency surgery for brain bleeding. X2, which reaches nearly 80mph and rotates riders’ seats, has been closed since 12 July, though Six Flags has not publicly explained the closure or commented on the investigation.