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. The garrison announced that a number of services would be closed or curtailed, and youth centers had cancelled off-post field trips. Other facilities in Germany such as Ramstein Air Base urged service members to "remain vigilant" on social media.
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."
There were indications of a threat as early as Friday, public messaging points from EUCOM at the time said efforts to stress vigilance were being increased "(d)ue to a combination of factors impacting the safety and security of US service members and their families stationed in the European theater," a defense official said.
"Malign actors have increased the threat of attacks against various non-military targets in Europe," the messaging points said, adding that EUCOM "takes operational security seriously, and reminds its employees to remain vigilant against attempts to collect on US activities, be extra mindful of conversations in public, and maintain a low profile so as not to be identified as a mark."
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.
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 US 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."