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Shares of Fiat Chrysler fell more than 7% on Monday as an investigation into its vehicles widened.

The US National Highway Traffic Safety Administration (NHTSA) doubled its investigation to cover 856,000 Jeeps, but has not requested a recall.

The investigation focuses on the cars' electronic gear shift, which lights up when moved from one gear to another, rather than making a physical click.

Some say this means drivers cannot tell whether or not it is in drive mode.

A Fiat Chrysler spokesman said the company "is cooperating fully".

The covers 2014 and 2015 Jeep Grand Cherokees and 2012 through 2014 Dodge Charger and Chrysler 300 sedans.

According to the NHTSA, drivers' inability to tell whether the car is in drive mode or not has allegedly led to 117 crashes.

The NHTSA is assessing "the scope, frequency, and safety-related consequences of the alleged defect".

In July, Fiat Chrysler was forced to recall 1.4 million Jeeps after it was revealed that the car's computer systems could be hacked. The company faced a record $105m (£72m) fine for failing to recall those cars from the road sooner.

The current investigation is not related to those issues, but has rattled investors.

Fiat Chrylser is also facing a lawsuit from US dealers alleging it inflated sales.