Several U.S. 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 U.S. military personnel or facilities, according to two U.S. officials.

The bases, including the U.S. Army garrison in Stuttgart, Germany where U.S. 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 U.S. Army.

One of the U.S. 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 U.S. 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 U.S. 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 theatre 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.

The German government brought in 580 international police officers to assist with security alongside German officers.

Interior Minister Nancy Faeser said ahead of the tournament, “We are preparing ourselves for all conceivable dangers: from Islamist terror to violent criminals and hooligans.”

France has also prepared for the possibility of a potential terror threat to the Olympic Games, which begin in just under a month. Since March, France has raised its national security alert system to the highest level, according to the U.S. embassy in France.

“French authorities actively monitor terrorist threats from organized groups and radicalized individuals,” the embassy said. “Attacks may happen with little or no warning, targeting tourist locations, major sporting and cultural events, and other public areas that attract large numbers of civilians.”