The court launched the investigation into metro line C over concerns surrounding the foundation of the Colosseum, which the subway runs near, and the rising costs of construction, Italy's ANSA news agency reported.
The investigation comes "following our serious concerns announced April 12," said the Italian heritage and environmental group Italia Nostra, adding that the "chance that the stability of the Colosseum could be at risk, that construction costs could rise, and that the economy and tourism to the city could be damaged."
Construction of the line began in 1990 and was scheduled to be completed by 2000, but has been halted multiple times. Administrators said this is partially due to the number of artifacts discovered underground during the construction that have had to be excavated before construction can proceed.
So far, the subway has cost Rome more than $6.5 billion -- nearly $4 billion more than than was initially forecast for the project.
"It's not the fault of one single project or of the person who managed it. This project includes every challenge imaginable -- institutional, procedural and technical," Audit Court President Luigi Giampaolino said last year.