U.S. military bases in Europe on heightened alert. Here's what to know.

American military bases in Europe are on heightened alert, according to U.S. officials, amid growing concerns of a possible terrorist attack in the coming weeks. Let's bring in CBS News correspondent Lily Luciano following this for us. Lilia, what's causing this concern? Is it typical for this time of year around the July 4th holiday? Well, Lindsey, there's actually a lot happening in Europe at the moment on top of the usual flow with summer travel from Americans. You have Wimbledon here, you have Germany that's currently hosting the Euro Cup. The final of the Euro Cup will take place in Berlin on July 14th. Less than two weeks after that, you have the Olympics in Paris. So military bases in Germany in particular, including the headquarters of US European Command in Stuttgart, as well as other bases in Germany, some of their installations in Romania, Bulgaria and the Aviano Air Base in Italy are all under threat level Charlie. So that is the second level of protection. It goes Alpha, Beta, Charlie and Delta, with Delta, of course, being the most severe. What this means is that there was either an incident or intelligence that led defense officials to suspect some form of terrorist activity is, quote, likely. Quite imminent, but likely so. For military personnel and their families, that means security bases. The increase, the securities increase in bases, some services will be unavailable, some non essential personnel may be working remotely and this is all for the foreseeable future. They didn't give an end date or anything of that nature. This also means that officials are advising service members and their families to stay vigilant abroad, to keep an eye on the State Department's advisories, take extra safety precautions and support any suspicious activity. And of course, this also goes as you mentioned, it's July 4th for Americans abroad and here in their rather in the US. Just a few weeks ago, the FBI director, Christopher Wray, warned that the terrorism lights are blinking red again in the United States and compared this moment to the timeline leading up to 911. What a warning. Lilia, thanks for your reporting.