UEFA has launched an investigation into Paris Saint-Germain for potential financial fair play violations, European football's governing body announced on Friday.
UEFA's Club Financial Control Body monitors all clubs as a matter of course but still made a point to announce the investigation into PSG and said the summer acquisitions were behind their move.
"The investigation will focus on the compliance of the club with the break-even requirement, particularly in light of its recent transfer activity," a UEFA statement said.
"In the coming months, the Investigatory Chamber of the UEFA Club Financial Control Body will regularly meet in order to carefully evaluate all documentation pertaining to this case.
"UEFA considers Financial Fair Play to be a crucial governance mechanism which aims to ensure the financial sustainability of European club football."
UEFA is not scheduled to assess whether teams violated FFP rules this summer until the fall of 2018, when the 2015-16, 2016-17 and 2017-18 seasons are cumulatively reviewed, though it was not clear whether this investigation could move up that time frame.
PSG acknowledged the probe later on Friday, confirming that UEFA planned to "immediately ensure" that the club's accounts for the current season were in compliance as of June 30, 2018.
The French club said they were "very confident" that they are in full compliance, insisting that they "always operated in total transparency" and have the "upmost respect" for UEFA.
PSG deputy CEO Jean-Claude Blanc has already met with UEFA officials to demonstrate the deals with Barcelona and Monaco were conducted within FFP rules, the club said.
PSG triggered Neymar's €222m buyout clause to acquire him from Barcelona on Aug. 3, and sealed Mbappe's before Thursday's deadline on an initial loan in a deal that sources said is worth €180m plus bonuses.
After Neymar's signing for a price that more than doubled the previous record fee, PSG chairman Nasser Al-Khelaifi said he was unconcerned about potential UEFA sanctions, the most significant of which would be exclusion from the Champions League.
But PSG were at least trying to work within the FFP system when they structured Mbappe's deal to begin with an initial loan, which could allow the club time to balance their books.
Since FFP rules allow a club to run only a €30m deficit over three years, PSG wanted to structure their deal for the 18-year-old so the financial impact of his arrival would not be taken into account this season.
The only other significant purchase PSG made this summer was left-back Yuri Berchiche from Real Sociedad for a reported €16m. They were able to sell off four players -- Serge Aurier, Blaise Matuidi, Jean-Kevin Augustin and Youssouf Sabaly -- for a reported €62m, but that still leaves PSG's net spending this summer at around €356m.
However, FFP rules say transfer fees are written off over the life of a contract, which means PSG will have to deal with only one-fifth of Neymar's five-year contract when weighing their expenditures against their revenue this season.
In their response to the investigation, PSG said they had already written off €104m including wages, this summer, and hinted that more players were likely to be sold before UEFA examines its books.
It said: "The club also reminds, if necessary, that it has under contract many high valued players allowing the club to generate very significant capital gains in the next two transfer windows (January and June) of 2018."
Qatar Sports Investments bought PSG in 2012 and have not been afraid to spend as they try to win the Champions League for the first time.
PSG previously fell afoul of FFP regulations and, along with Manchester City, were punished by UEFA in 2014. The Ligue 1 club were ordered to play past editions of the Champions League with only 21 players in their squad and spending on new signings was capped at around €55 million per season as punishment for breaching the rules.
They were also fined €20m a year for three years totalling €60m, but only the first year's fine was imposed. This summer was the first since they were hit with the FFP fines that they are totally out of FFP sanctions.
Even before Neymar's deal was completed, Barcelona suggested they would report PSG for a breach of FFP, a largely ceremonial and unnecessary move designed to put pressure on UEFA.