Federal regulators have begun interviewing individual specialists on the floor of the New York Stock Exchange (search) about an investigation of possible illegal trading, amid a leadership vacuum at the Big Board.

As the NYSE awaits the arrival next week of a temporary replacement for its ousted chairman, Richard Grasso (search), sources close to the Securities and Exchange Commission (search) said the agency's look at the probe is "a very high priority."

The SEC has been monitoring an NYSE inquiry into suspected trading violations by floor specialists, including possibly failing to stand aside in adequately liquid markets and possible "trading ahead" of customer orders, the sources said.

Specialists are being called in for private talks with SEC agents, marking a step forward in the SEC's review of whether the NYSE is "fulfilling its responsibilities to investigate specialists," said a source familiar with the matter.

Specialists manage the flow of most trading on the floor of the NYSE. Because they see buy and sell orders before anyone else on the floor, they know which way markets will go, giving them an advantage from which they are not supposed to profit.

But regulators are looking into whether specialists failed to hold aside their firms' own interests in markets with adequate buy and sell orders from customers. In addition, they are probing whether specialists put their own firms' interests ahead of customers' to profit from some advantageous trades.

The NYSE's biggest specialist firms have been involved in the NYSE probe. LaBranche & Co. Inc. (LAB), the biggest such firm, and the exchange have been locked in a public dispute over NYSE investigators seeking access to the firm's e-mails .

News of progress in the floor trading probe came just a day after John S. Reed (search), former Citigroup Inc. (C) co-chief executive, was named interim head of the NYSE. Reed was not expected to show up for work until next week.

Former NYSE Chairman Grasso resigned last week amid controversy over his $140 million pay and benefits package, fueling long-standing dissatisfaction among some at the SEC and in Congress about the NYSE's structure and management.

In the meantime, sources close to the SEC said the commission was expanding its role in leading governance and floor-trading reforms at the world's largest stock exchange.

Delivery to the SEC by the NYSE of a long-awaited report on its governance will be delayed, NYSE lead director H. Carl McCall (search) said on Sunday. He told Reuters the Oct. 2 deadline for the report would be postponed to give Reed time to weigh in on it.

An SEC spokesman declined to comment on the exchange floor trading probe, and on when the governance report was expected, what it will recommend and how the SEC views it.

But a source close to the SEC said, "Everything is on the table," including possibly splitting the NYSE's regulatory and business functions and separating the chairman and CEO jobs.

An NYSE spokesman was not immediately available for comment on the trading probe.