The Federal Trade Commission this week announced another set of probes to add onto the heaping mound of antitrust investigations the nation’s biggest tech firms now face. This time around, they’re digging into a decade’s worth of acquisitions that were small enough to escape scrutiny the first time around but may have proven to have big consequences after the fact.

The review will cover acquisitions made by Alphabet (Google), Amazon, Apple, Facebook, and Microsoft between 2010 and 2019, the FTC said. The probe is not a criminal investigation but rather a “wide-ranging study” to help regulators better understand what trillion-dollar companies are doing when they gobble up little startups and their staffs.

The smaller transactions escaped scrutiny the first time around thanks to the Hart-Scott-Rodino Act—the same law that mandates a look at bigger transactions. Under HSR, plans for mergers and acquisitions above a certain dollar threshold must be submitted to the FTC and Department of Justice in advance. The process is called, fittingly, premerger notification. Once a company has submitted its premerger filing, regulators have 30 days to take a look at the proposal and determine whether to probe deeper. If the waiting period expires or the FTC grants it early termination, the companies can move forward.