Law enforcement officials tell the Blotter on 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 Patch Barracks, the headquarters for U.S. European Command, 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, a former White House counterterror official.
The 9/11 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. Andrew 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.
A radical Islamist 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.
Late last month, the U.S. Embassy in Berlin issued a statement saying that security was being increased at the embassy and other consular facilities "in response to a heightened threat situation." The statement also urged "Americans in Germany to increase their vigilance and take appropriate steps to bolster their own personal security."
A spokesperson at Patch Barracks said they were not aware of any specific surveillance on the location, and they are not under any more specific surveillance than other U.S. facilities in Germany. "As a response to the U.S. Embassy Berlin's recent warden message, all U.S. military installations in Germany are urged to exercise increased viligance," they said in a statement to ABC News.
Watch the full report tonight on "World News With Charles Gibson."
This post has been updated.