Tesla Inc's (TSLA.O) Chief Executive Elon Musk said on Monday said that the electric-car maker was working on improving the much-awaited update to its self-driving software "as fast as possible."

The billionaire entrepreneur tweeted that the Full Self-Driving Beta version 9.2 is "actually not great imo (in my opinion), but Autopilot/AI team is rallying to improve as fast as possible."

"We're trying to have a single stack for both highway & city streets, but it requires massive NN (neural network) retraining."

Tesla had recently come under the scrutiny of U.S. safety regulators, who opened an investigation into its driver assistant system because of accidents where its cars crashed into stationary police cars and fire trucks.

Tesla CEO Elon Musk said on Monday said that the electric-car maker was working on improving the much-awaited update to its self-driving software "as fast as possible."

Two U.S. senators also called on the Federal Trade Commission to investigate Tesla, saying it misled consumers and endangered the public by marketing its driving automation systems as fully self-driving.