By Erik Kirschbaum

BERLIN, Sept 8 (Reuters) - Three suspected Islamist militants who were planning to attack U.S. installations in Germany had orders to act by Sept. 15 and knew police were hot on their trail before their arrest, a magazine said on Saturday.

The plan was foiled on Tuesday when police arrested two German converts to Islam and a Turk in the biggest German police operation in 30 years.

According to surveillance details published in Der Spiegel magazine, the men had been given a two-week deadline for their planned strikes in a late August call from northern Pakistan that was monitored by German police.

The militants, identified by German media as Fritz Gelowicz, Daniel Martin Schneider and Adem Yilmaz, had enough material to make bombs with power equal to 550 kilograms of TNT and were believed to be planning simultaneous car bombs across Germany.

Officials have said all three had trained in militant camps in Pakistan before forming a domestic cell of the "Islamic Jihad Union" -- a little known al Qaeda-affiliated Sunni Muslim group with roots in Uzbekistan.

According to Der Spiegel, Gelowicz and Yilmaz had mentioned "a disco filled with American sluts" along with airports, nightclubs or a U.S. military base as one target during a July 20 conversation that was bugged by police.

The three suspects were aware they were under close police observation, Der Spiegel said. At one point, one of the suspects got out of a car at a traffic light, calmly walked back to an unmarked police vehicle behind him and slashed its tyres.

The arrests were the culmination of an investigation that began a year ago, when U.S. officials alerted German authorities to e-mails intercepted from Pakistan.

U.S. President George W. Bush was closely following the case, the magazine reported. He asked German Chancellor Angela Merkel about it in June during a G8 summit in Heiligendamm.

The police launched the raid on Tuesday after two local traffic police officers unaware of the investigation stopped two of the suspects in a routine traffic control because their car had its bright lights on.

"Oh, they're on the federal police list," said one of the officers after running the names through a police computer in comments that were overheard by the suspects.

Authorities have said there are at least 10 people under investigation, including the three.

News of the arrest has shaken Germany, which has not suffered a major attack in past years. Germany refused to take part in the U.S.-led Iraq war but has some 3,000 troops stationed in Afghanistan and has been on high alert for attacks.

Die Welt newspaper on Saturday quoted security sources in Germany saying there might be as many as 49 suspects.

"We're not out of danger," Joerg Ziercke, president of the federal police office, told the newspaper.

Neither the federal police office nor federal prosecutors have commented on the details of the arrest or the probe.

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