Patrizia Reggiani Martinelli, 49, was arrested in an apartment in Milan. Another woman and three men were detained separately.
The arrest was carried out on the orders of the Milanese prosecutor Carlo Nocerino, who leads the inquiry into one of Italy's most mysterious crimes.
For Italians, it was yet another gripping chapter in a long-running saga of bitter feuds and ugly power struggles behind the family's chic image. Investigators allege that Gucci's elegant dark-haired former wife, with whom he had an acrimonious relationship, had paid hitmen more than half a billion lire to murder him. The couple had two children and reportedly squabbled over alimony.
Gucci, 45, was shot dead in broad daylight outside his office in central Milan by a well-dressed assassin who pumped four shots into him from close range before climbing into a waiting getaway car.
The other woman arrested was Giuseppina Auriemma, 51, a "spiritual adviser". Reuters