The Securities and Exchange Commission is investigating whether recent stock sales by Tesla chief executive Elon Musk and his brother, Kimbal Musk, violated insider-trading rules, according to people familiar with the matter.

The SEC’s investigation began last year after Kimbal Musk sold shares of Tesla valued at $108m, one day before the Tesla chief polled Twitter users asking whether he should unload 10% of his stake in the electric-car maker and pledging to abide by the vote’s results.