The U.S. military has issued a security alert at its bases in Europe, raising the threat level to the second highest.

The "Charlie" threat level signifies that a terrorist attack is likely. Officials told CNN that they have not seen a warning like this “in at least 10 years."

Security alert at U.S. bases

The alerts went out at the Army base in Stuttgart, Germany where US European Command is headquartered, as well as other bases in Germany and Italy, according to Stars and Stripes.

There are five threat levels, Normal, Alpha, Bravo, Charlie, and Delta, with Delta being the highest. 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.

While the exact cause of the alert wasn't shared, European officials have warned of possible terror threats this summer targeting the Paris Olympics and soccer championships in Germany.

Commander Dan Day of the U.S. European Command said the military 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.”

Biden fuels national security fears

National security concerns in the United States have been heightened by President Biden's disastrous debate performance last week, which has left the White House struggling to defuse concerns about Biden's age and fitness.

During the debate, President Trump blamed Biden for chaos on the world stage, including Russia's invasion of Ukraine and Hamas' October 7 attack on Israel. Trump also faulted Biden over a rise in suspected terrorists entering the U.S. through the Mexican border.

"We have the largest number of terrorists coming into our country right now. All terrorists, all over the world – not just in South America, all over the world. They come from the Middle East, everywhere. All over the world, they’re pouring in. And this guy just left it open," Trump said.

Biden was criticized for falsely claiming at the debate that no U.S. soldiers have died on his watch.

Biden's claim notably ignored the killing of 13 U.S. troops in a terror attack during the United States' frantic withdrawal from Afghanistan.