Three companies that use infotainment systems similar to the vulnerable one that allowed hackers to remotely hijack a Fiat Chrysler Jeep Cherokee last year were found to be safe, according to a 5-month-long investigation by the National Highway Traffic Safety Administration. These include Volkswagen (VLKAY), Audi AG (AUDVF), and Bentley.

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The original security flaw affected the Harman International-made (HAR) Uconnect infotainment systems installed in certain Fiat Chrysler (FCAU) vehicles. Fiat Chrysler recalled 1.4 million Jeep, Chrysler, Dodge, and Ram vehicles after two security researchers, Charlie Miller and Chris Valasek (both now at Uber), made their discovery public through a controversial Wired story.

Though Harman also supplies similar radio systems to other automakers, United States regulators determined that the hack affected only the one company. The others employed safety systems adequate enough to block hackers.

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Security architecture implementations in the head units supplied to other manufacturers are distinguishable from the Uconnect Access HUs provided to FCA. Audi AG and Bentley installed infotainment devices with countermeasures including multilayered security implementations and partitioned communication domains to reduce security vulnerability risks and mitigate or prevent cyber-attacks. Additionally, these other vehicles interacted with vehicle networks outside the infotainment system differently.

MORE: Here’s how to avoid your car getting hacked.

“Based on a thorough review of the technical information supplied in the course of this investigation, there does not appear to be a reason to suspect that the infotainment head units Harman supplied to other vehicle manufacturers contain the vulnerabilities identified by FCA,” a summary of the investigation concluded, referring to Fiat Chrysler Automobiles. “Accordingly, this investigation is closed.”