Adrian Newey, the man who designed the car Senna was driving, gave this quote: ” The honest truth is that no one will ever know exactly what happened. There’s no doubt the steering column failed and the big question was whether it failed in the accident or did it cause the accident? It had fatigue cracks and would have failed at some point. There is no question that its design was very poor. However, all the evidence suggests the car did not go off the track as a result of steering column failure… If you look at the camera shots, especially from Michael Schumacher’s following car, the car didn’t understeer off the track. It oversteered which is not consistent with a steering column failure.

https://www.formulanerds.com/features/the-story-and-conspira...

There's still debate on this.