Late last month (May 25, to be exact) shares of regional trucking firm Swift Transportation (NASDAQ:SWFT) jumped 20% after the company issued a press release that dramatically lifted second-quarter earnings guidance and outlined plans for an extended $40-million stock buyback. I covered the story, noting among other things, CEO Jerry Moyes' purchase of a large block of Swift stock during the quarter.

Thanks to the open dialogue and exchange of ideas we enjoy here at the Fool, it wasn't long before a reader pointed out the suspicious timing of Moyes' purchase (more on this later) in relation to the formal announcement. I've never been one to implicate somebody without due cause, and I wrote the timing off as a circumstantial, isolated incident. Unfortunately, new evidence has surfaced that has all the earmarks of being incriminating.

Swift has a "blackout period" that prevents insiders from trading company stock up to three weeks before a scheduled quarterly earnings release. Apparently, the policy does not apply to mere earnings revisions. Swift's board met on May 20. According to an SEC filing, Moyes then purchased 187,000 shares between May 21 and May 24. You'll recall the press release raising guidance and the subsequent 20% rise in Swift's stock occurred on May 25. Moyes' profit on these trades (excluding 7,000 purchased on May 19 that were omitted from the SEC filing) was well in excess of half a million dollars. It gets worse.

Earlier in the year, Moyes made a similar move with the purchase of 150,000 shares on March 22. Within 24 hours, Swift announced significant progress toward its $100-million stock repurchase commitment (the company had purchased 3.5 million shares for $65 million), lifting the stock enough to generate a $132,000 gain for Moyes. It still gets worse.

Back in March (March 16, to be exact), Swift ran into a few problems and was forced to dramatically lower guidance. Shareholders reacted quickly to the pessimism, sending shares 14% lower that afternoon. Fortunately for Patrick Farley, a Swift executive vice president, stock options for 100,000 shares had been exercised and promptly sold on March 9, thereby preserving a $1 million gain. Farley retired later that month.

In all fairness, these trades certainly could have been coincidental, but it's hard not to be skeptical given Swift's history of questionable corporate governance practices, such as the affiliation with some of Moyes' privately owned entities. Also disconcerting is the fact that Moyes owns nearly 35% of the company, and virtually all of his shares are currently being pledged as collateral for undisclosed loans. Should a plummet in Swift's shares precipitate the banks to call in these loans, I shudder to think of what the forced liquidation of 27 million shares would do to a stock with an average daily volume of only 728,000.

For now, Moyes' profits will be held in a trust account while the board's independent directors hire attorneys to review the trades. In the meantime, Swift's directors have taken concrete steps to monitor insider trading. The SEC, without a doubt, is scrutinizing this situation carefully; I suspect there will be more to follow.

Fool contributor Nathan Slaughter owns none of the companies mentioned.