An open air tour bus in San Francisco crashed on Friday in what looked like a situation where the huge bus just lurched and careened out of control before crashing to a stop. The bus ran down someone on a bicycle, as well as two pedestrians, and slammed into several cars and some construction site scaffolding before coming to a stop.

The accident and resulting chaos occurred in the city’s Union Square, reports The New York Daily News. So far, 20 people have been seriously injured with six of them being in critical condition. There were 30 people on board the bus that suddenly raced out of control during one of the busiest times of the day leaving destruction and chaos everywhere.

The tour bus made a huge crashing sound when it finally stopped after running into the scaffolding, sending people scattering. Police and emergency crews on the scene were on top of the bus trying to pull people from the wreckage. People were laid in the street and attended to immediately by paramedics in an effort to stabilize them before transporting them to nearby hospitals. The crash happened slightly before 3 p.m. Pacific time.

Police have not yet determined what caused the tour bus to roar out of control. They will be holding follow up investigations to determine if it was driver error or a mechanical failure of breakdown with the vehicle. Three men and three women were listed as the severely and critically wounded who were immediately transported to nearby San Francisco General Hospital. The ranged in age from 20-60.

Witnesses stated that the bus sped out of control for almost three blocks. Many said that the bus didn’t slow down at all until it crashed into the construction scaffolding. It was a wonderful late fall day in San Francisco and the sidewalks were full of shoppers and tourists and the cafes and restaurants were packed.