Nicholas Latifi says he was unaware that he made contact with Nikita Mazepin until after the race at Imola, having initially believed he crashed out by himself.
Latifi trailed Williams team-mate George Russell through the opening lap in wet conditions but spun on the entry to Acque Minerali.
Latifi recovered but on the following straight brushed wheels with Haas driver Mazepin and speared heavily into the barriers, retiring from the race.
The Canadian explained that he did not realise Mazepin had dropped behind pit-lane starter Sebastian Vettel on the opening lap, and thought the Aston Martin driver was the last car.
“Obviously visibility was extremely, extremely poor, probably the worst I’ve ever had at the start of the race,” said Latifi.
“The first off I had I caught a bit of rear locking… so I was off the track at the bottom of the Acque Minerali corner, came back on, staying to the left side as much as I could.
“I thought Vettel was the last driver as I saw him go by me… I came back on the track, naturally the line takes you there anyway, obviously very, very slippery.
“To be honest I didn’t even see Nikita, there was spray and looking back at the video he would have been in my blind spot, but was not aware he was there, so actually up until quite a lot [later] after the incident I thought I just got a bit of wheelspin and spun off because of that.
“I was not aware I made contact with Nikita until a lot afterwards.
“I’m extremely frustrated with myself that I didn’t even get to participate in the race and see what could have been possible. That’s how racing goes sometimes. Saturday was loving it, Sunday not so much.”
Russell’s subsequent crash meant Williams endured its first double retirement since the 2019 Russian Grand Prix.