After the Federal Communications Commission reversed its longstanding pattern of making moves that are friendly to Sinclair Broadcast Group and flagged “serious concerns” about its pending merger with Tribune Media, the stocks in both companies took double-digit dives.

Tribune Media, which resumed trading after a late-morning halt, closed at $32.11, down 17% on volume that was more than 10 times normal levels. Sinclair shares tumbled nearly 12%, ending at $29.10 on five times the usual volume. Shares in other major station groups such as Tegna and Nexstar also fell sharply on the chilling effect of the comments.

Sinclair said in April that it would divest 23 television stations to win approval, but noted in regulatory filings that although it planned to sell the Chicago and New York stations to third parties it would enter into agreements to run them. Those “sidecar” deals have become increasingly common in the local TV sector, but have drawn scrutiny. FCC chairman Ajit Pai, who has scoffed at the notion he treats Sinclair with kid gloves, stunned many media observers by saying the sidecar deals would effectively preserve control of the stations for Sinclair (already a massive power in local TV) “in practice, even if not in name.”

Pai said he would circulate a draft order for a vote by the five commissioners on the Republican-controlled body. The order would recommend that the matter be referred for administrative review. While the vast majority of deals do not survive these kinds of reviews, a Reuters report said the order will include a timeline during which the review must be completed. Should the transaction fall apart, the big-city properties of Tribune (in major markets such as New York, Los Angeles and Chicago) will make attractive properties for another buyer.

Sinclair first proposed the transaction in the spring of 2017, and initially it seemed certain to be approved despite potentially extending the company’s reach to more than 70% of U.S. households, well north of the traditional 39% ownership cap. Citing changes in the media landscape, the FCC has been reassessing the 39% cap and has signaled a possible easing or even an outright elimination of it. Many observers predict the cap will be boosted to at least

Several companies, Democratic politicians and trade groups piled on after Pai’s comments. “It is clear that the parties should abandon their proposed merger,” said the Communications Workers of America in a statement. (The CWA is one of multiple entities which filed a petition earlier this year seeking to block the deal.)

Added Jeff Blum, senior vice president of Public Policy and Government Affairs for Dish Network: “We are pleased that Chairman Pai has circulated an order designating the proposed Sinclair-Tribune merger for a hearing. It is a prudent step to closely scrutinize whether the proposed merger serves the public interest and consumers.”