FIA also probing secret Ferrari tire test (Update)
Both Ferrari and Mercedes are facing a FIA disciplinary inquiry after taking part in private tests with Pirelli earlier this season.
Ferrari ran in the wake of the Bahrain Grand Prix while Mercedes were in action in the days following the Barcelona race.
"The FIA has asked Team Mercedes AMG Petronas F1 and Scuderia Ferrari Team which have taken part in tire tests in the 2013 season to reply to a disciplinary inquiry in pursuance of the FIA Judicial and Disciplinary Rules," said an FIA statement earlier in the week.
"This follows the Stewards' Report from the Monaco Grand Prix and represents supplementary information required by the FIA in the light of the replies received from Pirelli, who were asked for clarifications on Tuesday May 28th."
A Ferrari spokesman told Autosport: "In reference to the numerous comments and interpretations circulated in recent days, Ferrari does not wish to make any further comment.
"The situation is rather straightforward [and has been] from the moment that a team broke what is a very clear regulation.
"The fundamental question refers to the compliance with article 22 of the sporting regulations, which does not allow the use of a current or previous year's car for any kind of testing carried out during the season.
"This is the only substantial aspect. Everything else is irrelevant detail."
05/31/13 (GMM) Amid the controversy about Mercedes' 'secret' Pirelli type test, F1's governing body on Friday said it is also investigating Ferrari.
At Monaco, Ferrari and Red Bull launched a protest when it emerged that Mercedes had tested tires for Pirelli with its 2013 car and race drivers Lewis Hamilton and Nico Rosberg just after the Spanish grand prix last month.
But at the same time, it was rumored that Ferrari had similarly tested, after Bahrain, but with a two-year old car and test driver Pedro de la Rosa at the wheel.
The FIA said late on Friday that it is investigating not only Mercedes, but also the Ferrari test.
In a statement, the Paris based federation said it has asked both teams "to reply to a disciplinary inquiry in pursuance of the FIA judicial and disciplinary rules".