KARLSRUHE, Germany - Three suspected al-Qaida members were working on making a shrapnel-laden bomb in Germany to attack a crowded area such as a bus or a bus stop when they were arrested, officials said Saturday.

The law enforcement officials say the trio, who had been under surveillance for months, hadn't picked a specific target, but were experimenting with explosives and detonators before authorities swooped in and detained them Friday.

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The suspects include a Moroccan, a German with Moroccan citizenship, and a German with Iranian citizenship.

The attack was "still in the experimentation stage," anti-terrorism prosecutor Rainer Griesbaum told a news conference. Officials decided to arrest the three in the western cities of Duesseldorf and Bochum after tips picked up through surveillance indicated they might be getting close to being able to carry out an attack.

One member of the cell was overheard saying he wanted to "do a bus." The trio had been researching how to build a bomb packed with metal objects that they could set off in a crowded area, officials said.

The man described as the main suspect - identified only as Abdeladim El-K., a 29-year-old Moroccan citizen - left Germany in early 2010 and trained in an al-Qaida camp in Waziristan near the Afghan-Pakistan border, and returned last year to carry out the attack, Griesbaum said.

El-K. had at one time resided in Germany on a student visa but later returned illegally after abandoning his studies. Officials said they were not sure how he had re-entered Germany. He appeared before a judge and was charged with membership in a foreign terrorist organization.

The other two suspects, a 31-year-old with German and Moroccan citizenship and a 19-year-old with German and Iranian citizenship, were to appear in court later Saturday.

Officials said the trio had praised the recent bomb attack on a cafe in Marrakesh, Morocco, although they had no connection to that attack. Still, German officials feared they might take it as inspiration to strike.

The plot was described as part of the effort by al-Qaida's leadership in Afghanistan since the beginning of last year to carry out an attack in Europe.

The instigator of the German conspiracy was said to have received the assignment to carry out a bombing from a high-ranking al-Qaida member in early 2010.

Germany's top police official said despite successfully disrupting the plot, the nation remains a possible target of international terrorists.

"We cannot sound the all-clear," said Joerg Ziercke, president of the federal police.

Zierke said the timing of the arrests was "a difficult security calculation," as authorities had to weigh the value of continuing to observe them against ensuring they did not succeed in carrying out an attack.

Material seized during searches Friday was being tested, but the suspects had discussed using acetone and hydrogen peroxide as bomb-making materials, Zierke said.