As many as five or six U.S. air marshals are now assigned to each U.S.-bound flight from airports in Frankfurt, London and Manchester, England, because of fears terrorists might attempt a coordinated series of mid-air explosions, law enforcement officials has stated.

"We're afraid someone in the back is going to mix something or light something up, so air marshals are being placed strategically through the plane," said one senior law enforcement official with direct knowledge of the stepped-up security.

The stepped-up security on flights out of Britain's Heathrow, Gatwick and Manchester airports began about two weeks ago, based on intelligence reports that another al Qaeda hijacking plot was in the making, the officials said.

It was  reported last Friday that U.S. and German officials fear a new terror attack is in the advanced planning stages.

Security on all flights out of Germany was increased about two months ago, based on similar intelligence, the officials said.

U.S. officials said the federal air marshals re-organized coverage of domestic flights and canceled some training classes to have enough marshals for the expanded coverage out of Germany and Britain.

A spokesperson for the air marshal program confirmed coverage in Europe had been increased "but not at the expense of protecting U.S. domestic flights."

U.S. officials said Germany law enforcement is providing full coverage of flights by Lufthansa and other German-based carriers flying into the United States. "They are stretched so thin they've had to rely on German military officers to help out," one U.S. official said. 

"The intelligence was that there are plans to take a plane and crash it in a high-density, high-profile place," one official told ABCNews.com.  The official said the timing and identity of the "high-profile place" was not contained in the intelligence reports.

U.S. officials said that an al Qaeda-connected cell in southwest Germany had been under 24-hour surveillance since the beginning of the year after some of its members were detected conducting surveillance on the headquarters of the U.S. European Command at Patch Barracks, near Stuttgart.

The cell, officials said, is made up of members of the Islamic Jihad Group, a violent terrorist organization based in Uzbekistan.

Two members of the cell were spotted outside Patch Barracks late on New Year's Eve, officials said.  When questioned, officials said they said "they wanted to see what U.S. soldiers do on New Year's Eve."

Further surveillance and intelligence reports led to stepped-up security at U.S. military and diplomatic facilities last month in Germany.

The U.S. Embassy in Berlin posted a warning that U.S. citizens should be vigilant because of a "heightened terror threat."

The official threat level in Britain is assessed as "severe," the highest level that signifies an attack is highly likely.  It has been at the "severe' level since an earlier hijacking plot was revealed last Aug. 14, 2006.