Multiple U.S. military-connected bases had a heightened state of alert activated out of caution, with multiple bases raising their threat levels to "Charlie" until further notice, Stars and Stripes reported.

The bases that increased their threat levels to Charlie include European Command headquarters in Stuttgart, Germany and the Aviano Air Base in Italy.

The U.S. Army saysthe Charlie level "applies when an incident occurs or intelligence is received indicating some form of terrorist action or targeting against personnel or facilities is likely." In these cases 100% of the individuals moving into and out of a base or around bases are subject to 100% ID card checks.

The next level above Charlie is "Delta" which indicates an attack has happened in that area.

A spokesperson declined to comment to Scripps News on any specifics about protection levels but European Command said there are a variety of factors that play into U.S. military community safety.

CNN reported that a U.S. official said they haven't seen this threat level in at least a decade at the base in Europe.

The European Soccer championships are currently underway in Germany, and Paris is hosting the Olympics in neighboring France. All massive events that bring with them heightened security because of crowd sizes and other issues.

European Central Command said in a statement obtained by Stars and Stripes that it is "constantly assessing a variety of factors that play into the safety of the U.S. military community abroad. As part of that effort, we often times take additional steps to ensure the safety of our service members."