European Union member states should close their borders until the continent gets the security situation under control, retired U.S. Army Col. Jack Jacobs said Tuesday.
"Once you are in Europe, the first place you land in Europe, you're wherever you want to go in Europe," he told CNBC's "Squawk Box." "It's been astonishing for decades, how the Europeans could be as complacent as they have been about security when the borders are completely and totally open."
More than 400 million EU citizens, as well as non-EU nationals, business travelers, and tourists, are able to travel throughout 26 European countries — known as the Schengen area — without having their passport or visa checked.
Maintaining security and holding the European Union together is a huge challenge with significant implications for U.S. multinationals, said Joseph Quinlan, chief market strategist for U.S. Trust of America Private Wealth Management.
"The European Union, the economy is $18 trillion. If it does become more difficult to move goods, people, and so forth, that's a big issue for the markets," he told "Squawk Box."
Michael Tyler, chief investment officer at Eastern Bank Wealth Management, said the attacks may feed fears in Britain about the perceived failure of European countries to integrate immigrants. At the time he spoke, no suspect had been identified.
Britain will hold a referendum on whether to stay in the European Union on June 23.
"I think it gives the exiters a little bit more momentum and a little bit more focus today," Tyler told "Squawk Box."