JEFFERSON COUNTY, Colo. (KDVR) — We now know more about a video that shows an 18-wheeler driver traveling erratically in the Colorado mountains shortly before it causing a deadly crash on Interstate 70.

That testimony came Wednesday in the trial of the driver, Rogel Aguilera-Mederos, who’s charged with the deaths of four people in the crash.

The man who shot the video, Pedro Olvera, said he posted it on Snapchat after his own close encounter with the truck. Little did he know that a short while later, that rig would cause explosions and heartache for so many people.

The video is harrowing. It shows an 18-wheeler coming down the mountains not far from the Buffalo Overlook on I-70.

“As I was approaching him to pass him on the slow lane, I was on his rear of his trailer, and he cut right in front of me. I had to brake, so I passed him, and we made eye contact and motioned watch what you’re doing,” Olvera said.

Olvera was one of many who testified Wednesday in the trial.

“I looked in my rearview mirror and saw a truck approaching fast,” Rand resident Casey Hull said.

Hull and his wife said they saw Medero’s truck hours before the crash in the Jackson County community of Rand.

“My husband had to pull the truck to the right side to avoid being hit, and the semi swerved in front of us,” Dawn Hull said.

A father, who on that day was teaching his daughter to drive, said he felt helpless seeing the 18-wheeler coming their way.

“I was screaming at my daughter … not a lot I could do but screaming at her to go, go, go, go,” Scott Nordby testified.

People who were hurt in the crash also testified, some fighting back tears as they relived that day.

The last witness of the day was a mechanic who worked on Mederos’ 18-wheeler in Houston.

Trial is set to continue on Thursday. Watch live on FOX31 NOW.