There is no indication on the web sites of any common ransomware that Garmin has been hit. However, such indications would normally be present only after a ransom demand has been made and rejected by the victim.
The website Bleeping Computer, which specialises in ransomware reporting, said Garmin had been hit by the WastedLocker ransomware.
The site said it had received confirmation from a source associated with the incident response that the outage was due to a WastedLocker attack.
The company said Evil Corp had previously been associated with the Dridex banking trojan and BitPaymer ransomware, both of which are believed to have earned their creators tens of millions of dollars. Two Russians alleged to be involved with the group have been indicted in the US.
In a short statement on its website on Thursday, Garmin said: "We are currently experiencing an outage that affects Garmin.com and Garmin Connect.
"This outage also affects our call centres, and we are currently unable to receive any calls, emails or online chats.
"We are working to resolve this issue as quickly as possible and apologise for this inconvenience."
The outage to the Garmin Connect site directly affects users because all the data that their device store is centrally stored.
The Taiwanese tech site iThome published an internal memo that Garmin had sent to its departments, warning that both its databases and servers had been affected due to a suspected hack.
The memo said production lines were being suspended.