The Senate’s Permanent Subcommittee on Investigations is probing corporate tax-cutting transactions and sent letters to at least 30 companies seeking details of past tax arrangements. Among them - Merck, Johnson & Johnson and Wyeth, according to The Wall Street Journal (subscription required). Merck and J&J tell the paper they’ll respond, but Wyeth wouldn’t comment.

The probe was apparently triggered by an accounting rule known as FIN 48, which took effect in January. The rule for the first time requires companies to disclose how much they have set aside to pay tax authorities if certain tax-cutting transactions are successfully challenged by the government, the paper writes. The disclosures require companies to attach a dollar figure to tax-savings arrangements they think could be vulnerable.

Although intended to inform investors, the disclosures also serve as a kind of road map for government authorities, guiding them to companies that may have taken an aggressive stance on their taxes. (In other words, maybe a little cheating). The FIN 48 disclosures generally reveal how much a company has set aside in an accounting reserve called “unrecognized tax benefits,” which reps the portion of tax benefits realized on a company’s tax return that also hasn’t been recognized in its financial reporting, according to the Journal.

Some companies that have received letters have made FIN 48 disclosures that stand out, the Journal writes. Merck, for instance, initially disclosed a potential tax liability of $7.4 billion, which was the largest absolute such figure reported by 361 large companies studied in a May report by tax and accounting analysts at Credit Suisse Group.