Company insiders sell more shares surrounding buyback announcements than on ordinary days, says a commissioner of the Securities and Exchange Commission, continuing a Washington, D.C. offensive against the practice.

SEC Commissioner Robert Jackson confirmed and expanded on earlier previous research conducted by his office the study in response to an inquiry from Sen. Chris Van Hollen, a Democrat from Maryland. Van Hollen had asked Chairman Jay Clayton if he was concerned that an earlier study prepared by Jackson’s office found twice as many company insiders sell their shares in the eight days after a buyback announcement than on an ordinary day.

Clayton told Van Hollen at a Senate Banking Committee hearing he thought the results may be coincidental, since that timeframe may coincide with when the window opens to permit executives to sell their shares.

To investigate Clayton’s comment about insider sales around buybacks being potentially coincidental, Jackson’s office extracted data on all buybacks between January 2017 and the end of 2018. It was necessary to estimate the length of any company pre-announcement trading prohibition, since every company has different policies. But even after accounting for these differences, the evidence showed that, on average, executives sell far more stock when they announce a buyback than on an ordinary day.

Jackson’s staff found that 38% of the firms had no trading in the thirty days prior to the date the buyback is announced. However, and consistent with prior research, the majority of firms conducting buybacks have insider transactions during the eight days after the buyback is announced.

In his written response to Van Hollen, Jackson wrote, “If executives believe a buyback is the right thing to do, they should hold their stock over the long term. Instead, we found that many executives use buybacks to cash out.”

That creates the risk, Jackson wrote Van Hollen, that rather than the long-term needs of investors, employees, and communities it’s insiders’ own interests that are driving companies’ buyback announcements.

When companies buy back their own stock back, the number of publicly traded shares outstanding is reduced. That makes earnings per share numbers look better and that makes shareholders and share-owning corporate executives very happy.

Corporations have been spending a larger and larger share of profits on dividends and corporate share repurchases. Jackson told Van Hollen that since January 2018, when the Tax Cuts and Jobs Act took effect, American public companies have announced a record $1 trillion in buybacks.

Sens. Chuck Schumer and Bernie Sanders recently wrote in an op-ed that share buyback are the manifestation of a corporate obsession with “maximizing only shareholder earnings to the detriment of workers and the long-term strength of their companies, helping to create the worst level of income inequality in decades.”

Related:The Sanders-Schumer buyback test would block almost all company stock repurchases

Van Hollen said on a conference call with reporters he is looking to introduce legislation on stock buybacks that would focus on encouraging executives to hold stock longer.

When executive insiders are aware of a pending buyback, they may be prohibited from trading until the announcement is public, says Jackson. His research found evidence that insiders can use buybacks as a chance to cash out at high stock prices—at the expense of long-term investors.

The SEC rules give a safe harbor, a type of immunity for enforcement actions for share price manipulation, to firms whose insiders sell when a buyback is announced. Jackson says those rules are outdated and give immunity from prosecution to executives that are essentially using the buyback to cash out.

The SEC said in 2015, in response to a request from Sen. Tammy Baldwin, Democrat of Wisconsin, that it could not track company share buybacks for compliance with the safe harbor provisions

The SEC did not respond to a request for comment about whether it was still unable to track company share buyback activity.