Several US military bases across Europe were put on a heightened state of alert over the weekend, with the level of force protection raised to its second-highest state amid concerns that a terrorist attack could target US military personnel or facilities, according to two US officials, writes CNN.

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

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 US Army. One of the US officials, who is 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”.

Asked about the shift, a spokesperson for US European Command, Cdr. Dan Day, declined to comment on specific force protection levels. But he told CNN that USEUCOM is “constantly assessing a variety of factors that play into the safety of US military community abroad. As part of that effort, we often times take additional steps to ensure the safety of our service members. For operational security reasons we will not get into specific measures, but we remain vigilant”.

Day added that “USEUCOM constantly monitors the security environment to ensure its personnel are informed and best postured to assure the safety of their individual person, family and loved ones. As always, USEUCOM advises personnel in the European theater to remain vigilant and stay alert at all times”.

It is not clear what intelligence triggered the heightened security, but European authorities have warned of a potential terror threat on the continent, especially ahead of the Paris Olympics in July and during the current European football championships in Germany.