Germany: ISIS Sleeper Cell Planned Suicide Bombings

Four Syrian nationals accused of being members of an ISIS sleeper cell are in custody after one of them revealed a plot to unleash suicide bombs on commuters in Germany, according to the country's federal prosecutor.

The plan, according to authorities, was to detonate two suicide bombs on a major street that is home to a busy public transportation hub in Duesseldorf in western Germany and then have at least one attacker unleash gunfire and explosives on bystanders. Planning for the attack went as far back as May 2014, when two of the men, identified as Saleh A., 25, and Hamza C., 27, received approval for the attack from ISIS leadership before slipping into Europe from Turkey through Greece, according to a statement on the prosecutor's website.

The two then arranged for the two others, Mahood B., 25, and Abd Arahman A.K., 31, to join them, authorities said. Abd Arahman A.K. is also suspected of making explosive vests for a rival terrorist group in Syria, the al-Qaeda-linked Nusra Front, before his involvement in the purported ISIS plot, the statement said.

The planned attack fell apart, however, when Saleh A. turned himself in to authorities in Paris in February and gave up the plot, the German prosecutor said.

German authorities announced today that in addition to arresting the three other members of the plot, they have issued an arrest warrant for Saleh A. and are seeking his extradition from France. Saleh A. is not Salah Abdeslam, one of the suspects in the November Paris attacks, who was arrested in Brussels in March. The Paris prosecutor has not responded to a request for comment from ABC News.

German authorities said the men's plot was not far along and no "concrete" steps had been taken to carry it out, but European counterterrorism officials have long been concerned about ISIS' reach into major European cities after deadly attacks in Paris and Brussels. The Syrian-based terrorist group has released several propaganda videos threatening Germany.

"[ISIS] routinely releases material in German, and there are a substantial number of German fighters," J.M. Berger, a co-author of "ISIS: State of Terror," told ABC News.

Dick Clarke, former White House counter-terrorism advisor and current ABC News consultant, said, "This is what ISIS is good at."

"Finding people, training them, sending them into Europe, having them act as sleeper cells and then activating them perhaps as much as two years later," Clarke said. "ISIS is a very capable organization that plans for the long game and has probably lots of sleeper cells scattered throughout Europe."

ABC News' James Gordon Meek contributed to this report.