A group of shareholders in Sinclair Broadcast Group on Tuesday asked the broadcaster’s board to launch an independent investigation into whether two executives and a board member engaged in insider trading earlier this year when they sold Sinclair shares ahead of news of weakening financials that triggered a slide in the company’s stock.

The request, made in a letter faxed to Sinclair CEO David Smith, is a prelude to a lawsuit that is expected to be filed in the coming weeks. In addition to the request for an investigation, the shareholders, led by the 1199 SIEU Greater New York Pension Fund, are also demanding that the three Sinclair officials return to the company the $18.5 million in proceeds generated by the stock sales.

The lawyer for the shareholders, William Lerach, declined to name the other shareholders involved, other than to say there are three or four others.

The shareholders’ demand deepens the intense scrutiny being leveled at the 62-station company since it announced plans to air a controversial program critical of Democratic presidential candidate John Kerry in the days before the Nov. 2 election. The controversy has taken a toll on Hunt Valley, Md.-based Sinclair’s already languishing stock. Shares in Sinclair fell 8 percent Monday, and dropped another 3.5 percent Tuesday.

Also on Tuesday, New York State Comptroller Alan Hevesi sent a letter to Sinclair challenging the company’s plans to air the documentary, “Stolen Honor: Wounds That Never Heal,” on the grounds it would further erode the value of the New York State pension fund’s investment in Sinclair.

The allegations, made by Mr. Lerach, center on stock sales made in late 2003 and early 2004 by Sinclair VPs Frederick Smith and J. Duncan Smith and director Robert Smith.

According to the letter sent to David Smith, Frederick Smith sold 75,000 Sinclair shares, or 15 percent of Sinclair holdings, on Dec. 23, 2003, and generated more than $1 million in proceeds, while Robert Smith between Dec. 30, 2003, and April 7, 2004, sold nearly 1 million shares-which the letter described as “virtually 100 percent of his holdings”-at a price of more than $13 a share, generating proceeds of $13.2 million. J. Duncan Smith, meanwhile, sold nearly 300,000 shares-also described as “virtually 100 percent of his holdings”-at an average price of more than $14.30 a share for proceeds of nearly $4.5 million.

Since the start of the year, Sinclair shares have tumbled more than 50 percent amid comparatively weak performance by the company, leading Mr. Lerach to believe that Sinclair officials knew their businesses were weakening due to mismanagement and that the three Sinclair officials sold their stock in anticipation of a stock-price decline.

A Sinclair official did not return a call seeking comment.

Mr. Lerach alleged that the three men sold their shares at or near the 52-week stock-price peak, and did so knowing that Sinclair’s financial performance would weaken in the coming months.

The discovery of the stock sales came after Sinclair earlier this month warned Wall Street that its third-quarter revenue growth would be smaller than expected. In its warning, Sinclair officials said weakness in the automotive advertising sector, coupled with the impact of a string of hurricanes in Florida, delivered a one-two punch that would result in a smaller increase in third-quarter revenue.

Once Sinclair announced its plan to run “Stolen Honor,” the shareholders, already “extremely unhappy,” according to Mr. Lerach, foresaw things becoming even worse. That’s when the shareholders, all clients of Mr. Lerach’s law firm before this latest dust-up, asked him to look into the situation with Sinclair. It was then, he said, that he discovered the stock sales and formed the view that the insiders had engaged in self-dealing.