MILAN  It is not the first time that Prime Minister Silvio Berlusconi of Italy has gotten into trouble for dallying with under-age girls, but the most recent scandal involving the septuagenarian’s private life could open the door to a full-fledged government crisis.

At the heart of it all is a Moroccan girl nicknamed Ruby, who turned 18 on Monday, but was still a minor last May when she was held in a police station in Milan, accused of theft until Mr. Berlusconi called and demanded she be released, Italian newspapers reported.

The details of what happened that night are now in the cross hairs of Milanese prosecutors, who must determine whether laws were broken or procedures ignored when Ruby was placed into the custody of Nicole Minetti, a former showgirl and Mr. Berlusconi’s dental hygienist. In March, Ms. Minetti was elected to the regional assembly in Lombardy as a candidate of the prime minister’s People of Freedom Party.

But beyond any legal repercussions, which may well not exist for Mr. Berlusconi, 74, the political stakes became apparent during the weekend as Gianfranco Fini, a disgruntled former ally, suggested that the prime minister might have overstepped his authority by intervening in Ruby’s release.