PARIS — In the dark early hours of an October morning in 2009, acting on an anonymous tip, police officers in the French city of Mulhouse picked up an elderly German doctor who had been left — bound, beaten and bleeding — in a street near the municipal courthouse. The man, Dieter Krombach, had been kidnapped outside his home in Germany and secreted across the border into France, where there was a warrant for his arrest in connection with the death of a French girl nearly three decades ago.

Earlier that morning, near Toulouse, André Bamberski lifted a ringing telephone. A voice informed him that the doctor was in Mulhouse. Mr. Bamberski gathered his things and left immediately. He took with him the 20,000 euros (then about $29,800) he had promised the abductors.

Mr. Bamberski, 74, will soon be tried for his involvement in the kidnapping. But the events of that morning were a victory for him, the culmination of nearly 30 years spent in obsessive pursuit of Dr. Krombach, the man who seduced and married his former wife, split his family and, Mr. Bamberski and the French authorities contend, raped and killed his 14-year-old daughter, Kalinka, in 1982.

In the years after the girl’s death, the doctor came to be known in Germany as a sexual predator; in 1997 he was convicted of drugging and raping a teenage patient. Dr. Krombach, now 76, had nonetheless lived largely untroubled in Germany, safe behind a German refusal to extradite him for trial in France.