Two people who work for the highway service company, Autostrade, are also under investigation for poor guardrail installation and upkeep, the prosecutor Rosario Cantelmo told Italian media.
Lametta is the brother of Ciro Lametta, the driver who died in the crash.
The bus had been making its way back from a pilgrimage to Pietrelcina, with 48 people on board, when it careered off the road and plunged 30 feet down a slope.
Investigators probing the state of both the coach and the US-imported Jersey concrete barrier which shattered on impact, leaving the bus to sail over the edge.
The crash happened in an area known as an accident black spot and investigators are also looking into whether the barrier was high enough to withstand impact with larger vehicles.
The vehicle, made in 1995, had more than 800,000 kilometres (500,000 miles) on the clock and bits of the transmission were reported to have fallen off before the accident, possibly slicing through the brake tubes, Italian media said.
Italy held a day of mourning on Tuesday for the victims, as relatives wept over their coffins at a mass funeral near Naples.
The coach crash was the deadliest in western Europe in the last decade and the worst in the continent since an October 2010 accident in Ukraine when 45 people died.