Several US military bases across Europe were put on a heightened state of alert over the weekend, amid concerns of a terrorist attack that could target US military personnel of facilities – according to two US officials cited by CNN.

“The level of force protection raised to its second-highest state amid concerns,” it added.

The US Army garrison in Stuttgart, Germany where the US European Command is headquartered raised their alert level to Force Protection Condition “Charlie” on Sunday – according to base officials.

That status “applies when an incident occurs or intelligence is received indicating some form of terrorist action or targeting against personnel or facilities is likely,” according to the United States Army.

One of the US officials stationed at a base in Europe, told CNN they haven’t seen this threat level “in at least 10 years,” and said it usually means the military has received an “active-reliable threat.”

The nature of the threat that triggered this heightened security is unknown, but European authorities warned of a potential terror threat on the continent.