The Federal Trade Commission has begun to warn some companies awaiting merger approval that their deals face further investigation and may be challenged even after an initial review period expires.

Citing a “tidal wave” of merger filings that are straining its resources, the antitrust agency said Tuesday that it’s telling companies doing deals that can’t be fully reviewed within the 30-day waiting period that the investigations remain open.

“Companies that choose to proceed with transactions that have not been fully investigated are doing so at their own risk,” Holly Vedova, the acting head of the FTC’s competition bureau, wrote in a blog post on the agency’s website.

The FTC said it’s preparing to send a letter to those companies pursuing deals that will get an extended review. The letter states that “inaction by the commission before the expiration of the waiting period should not be construed as a determination regarding the lawfulness of the transaction.”

The move comes after President Joe Biden issued an executive order last month calling on federal agencies to take steps to boost competition in industries across the economy and tackle the effects of years consolidation that has swept many markets.

FTC Chair Lina Khan told Congress last week that antitrust officials are processing the highest number of merger filings in two decades and are struggling to keep up with the workload. Companies have announced about $2.8 trillion of deals so far in 2021, an unprecedented number that puts this year on track to be the most active ever, according to data compiled by Bloomberg.

“I am deeply concerned that the current merger boom will further exacerbate deep asymmetries of power across our economy, further enabling abuses,” Khan told lawmakers.

Under the U.S. system for reviewing mergers, deals are subject to an initial 30-day review during which the companies cannot close. After that period, the FTC or the Justice Department, which shares antitrust jurisdiction, can open an in-depth investigation. If they don’t, the companies are free to close.