The U.S. Treasury Department sent notices to ship management companies about more than 100 vessels it suspects may have violated the price cap on Russian oil imposed after the invasion of Ukraine, a person familiar with the matter said.

The notices went to companies in about a dozen countries, according to the person, who asked not to be identified discussing private communications. Treasury recently imposed its first sanctions on companies accused of violating the price cap, and the latest notices suggest it may begin to hold companies to account more often.

A Treasury spokesperson said that the department doesn’t confirm or comment on investigations or enforcement actions but is committed to enforcing the cap. The news was first reported by Reuters. The notices generally ask companies to attest that they bought oil below the cap or risk facing penalties.

The Group of 7 imposed a $60-a-barrel cap on crude purchases in December and other thresholds for refined fuels in February. The measures require shipowners to get attestations from traders that the oil didn’t breach the limit. If it did, they’re not allowed to provide services.

But oil is now trading well above the price cap and Russia has built up a large shadow fleet of tankers while shifting its exports to countries such as India.