Defense Secretary Ash Carter says it appears a navigational error caused Navy boats to stray into Iranian waters in the Persian Gulf where they were detained overnight by Iran.

In an interview Thursday with Univision, Carter said based on information commanders have provided, the boats accidentally strayed into Iranian waters due to a navigational mistake. Less than a day after being detained, the sailors were back with their American fleet.

Carter says the sailors were not on a covert mission and were simply making their way from Kuwait to Bahrain, both on the western coast of the Gulf.

Defense officials say it remains unclear if the crew made a mistake or if the navigational equipment was to blame for the incident.