Law enforcement officials tell ABCNews.com that U.S. air marshals have been diverted to provide expanded protection of flights between Germany and the United States.
"The information behind the threat is very real," a senior U.S. official told ABC News.
German Interior Minister Wolfgang Schauble told reporters, "The danger level is high. We are part of the global threat by Islamist terrorism."
Of particular concern, according to U.S. and German law enforcement officials, is the U.S.-European Command at Patch Barracks, near Stuttgart.
Security at all U.S. military and diplomatic facilities has been increased in the last month, following reports that suspected terrorists had conducted surveillance of the Patch Barracks facility.
"The attack would be designed to create high numbers of casualties among both Germans and the U.S. military," said ABC News consultant Richard Clarke, the former White House counterterror official.
The 9/ll hijackers planned their operation out of Hamburg, Germany, and the country continues to be known as a staging area for al Qaeda and groups affiliated with it.
"There are 300 to 500 people who are suspected to be part of al Qaeda cells in Germany," said Col. Andrews Pratt (Ret.) of the George Marshall Center in Germany.
"In a democratic state like Germany, you just can't go out and arbitrarily arrest people because they are under suspicion," Pratt said.
German officials have called for enhanced police powers to keep suspected terror groups in check.
Several radical Islamist groups have threatened violence unless Germany withdraws its troops from the NATO force in Afghanistan.
An al Qaeda-connected group in Iraq took a German woman and her adult son hostage in February, threatening to execute them if Germany did not pull its troops out of Afghanistan. The two are still being held.