A man suspected of being the mastermind of a $300 million penny-stock trading scheme is in federal custody because the international flight he was traveling on from Canada to Mexico made a brief stop in Phoenix.

The layover on Tuesday at the Phoenix Sky Harbor International Airport gave F.B.I. agents enough time to arrest Gregg R. Mulholland and charge him with overseeing the manipulation of numerous penny stocks and then laundering the profits through accounts at no fewer than five offshore law firms.

Mr. Mulholland’s arrest caps a federal investigation that lasted more than two years and first came to light last September with the indictment of four men and an offshore investment firm in Belize, Legacy Global Markets. Federal prosecutors contend that Mr. Mulholland was the secret owner of Legacy Global.

One of the stocks Mr. Mulholland is charged with manipulating is Cynk Technology, a little-known company based in Belize that claimed to be in the social network business and whose market value soared briefly to $6 billion last summer. The spectacular and bewildering rise of shares of Cynk, a company with no revenue and a single employee, was the buzz of Wall Street last July. Securities regulators halted trading in the stock, suspecting it was being manipulated. The shares of the company plummeted 85 percent when trading resumed on July 25.