The FIA has asked Ferrari as well as Mercedes to supply information for a disciplinary inquiry over recent Pirelli tyre tests that have taken place at Barcelona.
Mercedes' secret test for Pirelli with its 2013 car immediately after the Spanish Grand Prix was the initial focus of the scandal when news of the test broke during the Monaco GP weekend.
Ferrari and Red Bull lodged a protest against the team using a current car, with the stewards referring the matter to the FIA.
It also emerged that Ferrari's customer car division, the Corse Clienti, provided a 2011 car for a tyre test for Pirelli two weeks before the Mercedes session.
On Friday night the FIA issued a statement saying it wanted both Ferrari and Mercedes to participate in its investigation.
"The FIA has asked Team Mercedes AMG Petronas F1 and Scuderia Ferrari Team which have taken part in tyre tests in the 2013 season to reply to a disciplinary inquiry in pursuance of the FIA Judicial and Disciplinary Rules," said the statement.
"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 28."
Although Mercedes' testing of a 2013 car has been the main focus of the row up until now, Ferrari's involvement in the disciplinary inquiry is more of a surprise because it was previously believed that its running of an '11 car was within the rules.
The exact details of the test have been kept quiet, but it is understood that the test with the F150 was paid for by Pirelli and run by the Maranello outfit's Corse Clienti division, so completely separate from the race team.
F1 teams have long been under the impression that running two-year-old cars was allowed, but there are now suggestions that even this could be in doubt.
Article 22.1 of the sporting regulations defines exactly what cars are exempt from the in-season testing ban - and appears to revolve around the definition of 'substantially different' from the current 2013 contenders.
"Track testing shall be considered any track running time not part of an event undertaken by a competitor entered in the championship, using cars which conform substantially with the current Formula 1 technical regulations in addition to those from the previous or subsequent year," states the rule.
One interpretation could be that a 2011 car is not 'substantially' different enough from the 2013 cars to be exempt from the testing limitations.