A charter bus swerved on the rain-slicked Interstate 15 and rolled down an embankment near the North San Diego County community of Fallbrook on Saturday morning, killing three people, leaving one 5-year-old in critical condition and injuring 17 others, authorities said.

There were a total of 21 people on the bus. Seventeen passengers were thrown from the bus, scattered throughout the area and one of the dead was trapped under the vehicle after it landed on its roof, California Highway Patrol at the scene confirmed.

“The charter bus lost control and went over the side of the embankment, it appears to be a solo vehicle accident,” Stephen Abbott Fire Chief with North County Fire said.

The incident was reported at around 10:29 a.m. on southbound Interstate 15 near State Route 76.

@NorthCountyFire is on scene of a bus rollover on southbound 15 south of 76 Highway. We have multiple people trapped and injured. Southbound 15 Freeway will be heavily impacted. #FreewayIC — North County Fire (@NorthCountyFire) February 22, 2020

Reaching the passengers was complicated for the rescuers as they needed to stabilize the bus to ensure it would not slide down the embankment, with erosion also playing a factor, Choi said.

It took officials 30 minutes once they arrived at the scene to extricate the people from the bus and transport them to the nearest hospitals.

Seventeen passengers were transferred to various hospitals with injuries ranging from life-threatening to non-life threatening. Three people, all women, were confirmed dead at the scene.

A 5-year-old boy who was initially taken to a local hospital has since been flown to Riverside University Health System in critical condition with a head injury, CHP said.

North County Fire Capt. John Choi initially said the bus did not have seatbelts. CHP later said the bus did have seatbelts, but it appeared that the majority of the passengers were not wearing seatbelts.

The driver of the bus suffered minor injuries and was not taken to a hospital. He is from the Whittier area, a city just south of Los Angeles, Oceanside CHP Officer Mark Latulippe said.

NBC 7's Niala Charles spoke to a man who witnessed the crash and stopped to help.

Christopher Williams said he witnessed the bus hydroplane from the third lane to the fourth lane, the slow lane. The bus then lost control and veered to the right about 200 yards in front of his car.

"The bus driver tried to turn the wheel and it hit an embankment and just flipped over, went down the hill," Williams told NBC 7. "[I] got out and just ran down the hill, started just picking people up and carrying them up the hill."

The bus was traveling from El Monte in Los Angeles County to San Ysidro and Tijuana, CHP said.

By Saturday night, the National Transportation Safety Board sent a group of four to investigate the crash.

Choi said traffic in the area would be heavily impacted for the next few hours. Lanes were reopened to traffic by around 4:40 p.m., Caltrans San Diego said.

A total of 10 ambulances, 10 fire engines, and one heavy rescue unit responded to the scene.

No other information was available.

Rescue crew were trying to recover the bus from the embankment along I-15. NBC 7's Allison Ash has more.