Several US military bases in Europe were put on high alert this past weekend over a possible terror attack in the region that could target American personnel, according to multiple reports.

The alert is the second highest state of alarm issued by the US Army, and an American official at one of the bases said they hadn’t witnessed that level in at least a decade.

The US Army garrison in Stuttgart, Germany, where the US European Command has its headquarters, was one of many bases that raised its status to the alert level, Force Protection Condition “Charlie,” on Sunday, officials told CNN.

Charlie “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 army’s policy.

Officials at EUCOM declined to elaborate on the heightened terror, but the office said in a statement that it’s “constantly assessing a variety of factors” in terms of security and safety.

“For operational security reasons, we will not get into specific measures, but we remain vigilant,” EUCOM said.

One US official who is stationed at a base in Europe told CNN that they have not seen the alert level raised to Charlie “in at least 10 years,” claiming such a move likely means the military has received an “active-reliable threat.”

According to Stars and Stripes, an independent military publication, in addition to several facilities in Germany, US army bases in Italy, Romania, and Bulgaria have also received the alert.

European authorities have been particularly vigilant over potential terror threats on the continent during the ongoing European football championships in Germany, with Berlin deploying 580 international police officers to assist with security.