Tesla shares fell after the National Transportation Safety Board said it sent investigators to look into a fatal car crash last week in California, according to a post on social media.

A negative analyst call from Citigroup also weighed on the stock, which has been under pressure this past one month.

The NTSB's post on Twitter said: "2 NTSB investigators conducting Field Investigation for fatal March 23, 2018, crash of a Tesla near Mountain View, CA. Unclear if automated control system was active at time of crash. Issues examined include: post-crash fire, steps to make vehicle safe for removal from scene."

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"We have been deeply saddened by this accident, and we have offered our full cooperation to the authorities as we work to establish the facts of the incident," Tesla said to CNBC.

Citi Research said Tuesday that its analysis of the company's Model 3 competition points to near-term risk for the stock.