(Bloomberg) -- The U.S. Federal Trade Commission has begun to warn some companies with proposed mergers under review 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 investigation remains open.

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