As a chemist at the Food and Drug Administration, which screens medicines for sale to the public, Cheng Yi Liang had access to the kind of information that can move drug company stocks.

According to the government, he has illegally traded on that information in advance of at least 27 FDA announcements since 2006, reaping profits or avoiding losses of more than $3.6 million.

Liang, 57, was arrested at his Gaithersburg home early Tuesday, along with his son Andrew Liang, 25, who is accused of participating in the scheme. The two face criminal charges, and the elder Liang also is named in a civil insider trading suit filed by the Securities and Exchange Commission.