Members of Finland’s National Bureau of Investigation were working to crack a drug smuggling ring, when they intercepted phone calls and text messages between a suspected supplier and buyer emanating from Pasila, a district of Helsinki.

One party to the communications, they suspected, was a notorious drug smuggler. The other they could not place. After months of more surveillance, prosecutors say, the officers were startled to discover that the mystery man appeared to be the head of the Helsinki police’s antidrug squad, a decorated officer, Jari Aarnio.

The discovery led to Mr. Aarnio’s arrest two years ago. Now, as details of the scandal pour out weekly from what Finland’s news media has called “the trial of the century,” the case is testing this otherwise orderly Nordic country’s once-solid confidence in its laws and institutions.

The deputy general prosecutor, Jorma Kalske, said in an interview that Mr. Aarnio, 58, a burly man once known as the Emperor among colleagues, used the lessons he gleaned in more than 30 years of fighting drug traffickers to become the leader of his own cartel. Mr. Kalske said Mr. Aarnio smuggled 10 million euros worth of hashish to Finland from the Netherlands.