It emerged separately that the advertising industry expects more top-level resignations today.
The exchange investigation centres on the 7 million Saatchi shares that were traded on Monday, mostly in the hours after the resignations of Jeremy Sinclair, acting chairman, Bill Muirhead and David Kershaw were tendered at noon and before the company confirmed their departure: just one minute before the close of dealings in the stock market.
Officials from the exchange yesterday contacted the company in connection with the recent heavy share trading. While the exchange declined to discuss the investigation, Charlie Scott, the company's chief executive, said: "I can confirm that the Stock Exchange has contacted us. And I can confirm that this company has done nothing wrong. I hope that the people who resigned did the same thing."
He added: "The Stock Exchange has asked us to explain the facts of yesterday [Monday], when the stock price went down before the announcement."
Maurice Saatchi, who last month was ousted as chairman of the company he founded, and his brother Charles have not been questioned by the exchange. Last week the brothers sold their entire holdings of 891,000 shares each at around 140p a share. A friend of the brothers said yesterday: "I don't think that it [Monday's dealings] involves them."
It also emerged yesterday that Saatchi had started to look for a new chairman immediately after Maurice was ousted. An industry source said that Saatchi was close to forming a short-list of candidates, and that it probably intended to make an announcement before the release of its next set of financial results in March. Mr Scott will be acting chairman until a successor is found.
Amid the speculation, Saatchi's shares continued to retreat yesterday. There was a strong rumour that another four senior executives in the advertising agency's London office were planning to resign.
A spokesman for Saatchi said: "I don't know of any more departures, but you may have been told before us . . . you were last time."
Mr Scott added that he believed that some of the rumours were part of an attempt to weaken the company. "It is fair to say that the focus of this situation appears to be directed at the London agency.
"It is all right for a bunch of heavy hitters to do what they want, but they are not doing it in the best interests of people who have been working with us for a long time."
Mr Scott also announced that the boardroom and executive gaps were being filled. Alan Bishop, chief operating officer in America, will replace Mr Muirhead as head of the US operations, and Michael Bungey, chief executive of the Bates Worldwide subsidiary, and Ed Wax, chief executive of Saatchi advertising worldwide, are being promoted to the main board.
An additional attack on the three who resigned on Monday was made by Saatchi's spokesman, who accused them of making their action known to the press before they informed the company. "We feel very let down by that."
That drew a terse response from the spokesman for the three men. "It is absolutely not true that we informed the press first. If Saatchi want to go into print about that then we will sue them because it is a total lie."
The market value of Saatchi has plunged from more than £340m to £246m since Maurice Saatchi was ousted. Shares, which lost 16p on Monday, fell swiftly from 124p to 108p yesterday, forcing the company into holding a conference with analysts to try to stopthe rot.
The conference did appear to have some success with investors. Shares recouped 4p of the early loss to close at 112p.
More than 5 million were shown as traded, although the final figure was probably much higher, with several large delayed transactions being registered before the close at up to 113p a share.
Mr Scott told analysts: "This company is still run by people who understand advertising and the other businesses Saatchi operates. I regret very much that the share price has taken a knock, and I believe it reflects a co-ordinated campaign to de-stabilise the company.
"I ask myself whether this is an attempt to do damage for its own sake or whether it is something which has been done with other motives.
"With regards to Maurice Saatchi's departure, the board took a long time to reach its conclusions because it was aware that Maurice might seek to damage the company if he were no longer chairman."
Mr Scott added that it was intended to change the Saatchi name of the holding company.
The company's attempts to placate investors also included releasing a letter of support from Procter & Gamble, the consumer products group and the company's biggest client.
The letter from Ross Love, head of advertising at P&G, said: "I want to confirm unequivocally that Procter & Gamble has no intention to move any brand out of the Saatchi & Saatchi agency, whether in the US or abroad because of the recent conditions."
Doubts remain, however, whether Mars, the confectionery group, will retain Saatchi. John Murray from Mars' marketing department said yesterday: "We are reviewing our options. We are talking with our existing and other agencies."
One analyst, who declined to be named, added support. He said: "Maurice Saatchi has less backing in the City than among his own employees. His chances of finding anyone in the City willing to put up the £350m he would need to buy the company are virtually zero."
The Saatchi brothers' friend, however, denied that they intended to buy the company. Additionally, Mr Muirhead issued a statement saying that "under no circumstances are we planning to buy the company".