ISTANBUL — Few here doubt that the case began with something threatening: in June 2007, 27 hand grenades and fuses were found in the attic of a house in an Istanbul slum. Investigators claimed they were stashed there by an ultranationalist retired officer and they were later linked to an elaborate coup plot.

But the question many are asking, inside and outside Turkey, is whether the Islamic-inspired government is exaggerating the threat in order to wage a much larger battle against this moderate Muslim nation’s secular establishment.

Since 2007, 300 people have been detained during the investigation of an underground group known as Ergenekon, including a writer of erotic novels, four-star generals and other military officers, professors, editors and underworld figures — some of whom appear to have committed no offense greater than speaking in favor of Turkey as a secular state...

... The case has brought into relief the larger strains in Turkey between a secular elite seeking to hold on to its waning influence and a growing, increasingly assertive population of observant Muslims. The case is being watched closely in Brussels, headquarters of the European Union, as a barometer of Turkey’s adherence to Western standards of justice. It comes as the country’s prospects for joining the bloc seem to be diminishing...