WASHINGTON — Last April, the chairman of the Federal Communications Commission, Ajit Pai, led the charge for his agency to approve rules allowing television broadcasters to greatly increase the number of stations they own. A few weeks later, Sinclair Broadcasting announced a blockbuster $3.9 billion deal to buy Tribune Media — a deal those new rules made possible.

By the end of the year, in a previously undisclosed move, the top internal watchdog for the F.C.C. opened an investigation into whether Mr. Pai and his aides had improperly pushed for the rule changes and whether they had timed them to benefit Sinclair, according to Representative Frank Pallone of New Jersey and two congressional aides.

“For months I have been trying to get to the bottom of the allegations about Chairman Pai’s relationship with Sinclair Broadcasting,” Mr. Pallone, the top Democrat on the committee that oversees the F.C.C., said in the statement to The New York Times. “I am grateful to the F.C.C.’s inspector general that he has decided to take up this important investigation.”

It was unclear the extent of the inspector general’s investigation or when it might conclude, but the inquiry puts a spotlight on Mr. Pai’s decisions and whether there had been coordination with the company. It may also force him to answer questions that he has so far avoided addressing in public.