I was shocked when I heard that Mary Jo White, a former U.S. Attorney and a partner for the white-shoe Wall Street defense firm Debevoise and Plimpton, had been named the new head of the SEC.

I thought to myself: Couldn't they have found someone who wasn't a key figure in one of the most notorious scandals to hit the SEC in the past two decades? And couldn't they have found someone who isn't a perfect symbol of the revolving-door culture under which regulators go soft on suspected Wall Street criminals, knowing they have million-dollar jobs waiting for them at hotshot defense firms as long as they play nice with the banks while still in office?

I'll leave it to others to chronicle the other highlights and lowlights of Mary Jo White's career, and focus only on the one incident I know very well: her role in the squelching of then-SEC investigator Gary Aguirre's investigation into an insider trading incident involving future Morgan Stanley CEO John Mack. While representing Morgan Stanley at Debevoise and Plimpton, White played a key role in this inexcusable episode.

As I explained a few years ago in my story, "Why Isn't Wall Street in Jail?": The attorney Aguirre joined the SEC in 2004, and two days into his job was asked to look into reports of suspicious trading activity involving a hedge fund called Pequot Capital, and specifically its megastar trader, Art Samberg. Samberg had made suspiciously prescient trades ahead of the acquisition of a firm called Heller Financial by General Electric, pocketing about $18 million in a period of weeks by buying up Heller shares before the merger, among other things.

"It was as if Art Samberg woke up one morning and a voice from the heavens told him to start buying Heller," Aguirre recalled. "And he wasn't just buying shares – there were some days when he was trying to buy three times as many shares as were being traded that day."

Aguirre did some digging and found that Samberg had been in contact with his old friend John Mack before making those trades. Mack had just stepped down as president of Morgan Stanley and had just flown to Switzerland, where he'd interviewed for a top job at Credit Suisse First Boston, the company that happened to be the investment banker for . . . Heller Financial.

Now, Mack had been on Samberg's case to cut him in on a deal involving a spinoff of Lucent. "Mack is busting my chops" to let him in on the Lucent deal, Samberg told a co-worker.

So when Mack returned from Switzerland, he called Samberg. Samberg, having done no other research on Heller Financial, suddenly decided to buy every Heller share in sight. Then he cut Mack into the Lucent deal, a favor that was worth $10 million to Mack.

Aguirre thought there was clear reason to investigate the matter further and pressed the SEC for permission to interview Mack. Not arrest the man, mind you, or hand him over to the CIA for rendition to Egypt, but merely to interview the guy. He was denied, his boss telling him that Mack had "powerful political connections" (Mack was a fundraising Ranger for President Bush).

But that wasn't all. Morgan Stanley, which by then was thinking of bringing Mack back as CEO, started trying to backdoor Aguirre and scuttle his investigation by going over his head. Who was doing that exactly? Mary Jo White. This is from the piece I mentioned, "Why Isn't Wall Street In Jail?":

It didn't take long for Morgan Stanley to work its way up the SEC chain of command. Within three days, another of the firm's lawyers, Mary Jo White, was on the phone with the SEC's director of enforcement. In a shocking move that was later singled out by Senate investigators, the director actually appeared to reassure White, dismissing the case against Mack as "smoke" rather than "fire." White, incidentally, was herself the former U.S. attorney of the Southern District of New York — one of the top cops on Wall Street . . .

Aguirre didn't stand a chance. A month after he complained to his supervisors that he was being blocked from interviewing Mack, he was summarily fired, without notice. The case against Mack was immediately dropped: all depositions canceled, no further subpoenas issued. "It all happened so fast, I needed a seat belt," recalls Aguirre, who had just received a stellar performance review from his bosses. The SEC eventually paid Aguirre a settlement of $755,000 for wrongful dismissal.

It got worse. Not only did the SEC ultimately delay the interview of Mack until after the statute of limitations had expired, and not only did the agency demand an investigation into possible alternative sources for Samberg's tip (what Aguirre jokes was like "O.J.'s search for the real killers"), but the SEC official who had quashed the Mack investigation, Paul Berger, took a lucrative job working for Morgan Stanley's law firm, Debevoise and Plimpton, just nine months after Aguirre was fired.

It later came out that Berger had expressed interest in working for the firm during the exact time that Aguirre was being dismissed and the Mack investigation was being quashed. A Senate investigation later uncovered an email to Berger from another SEC official, Lawrence West, who was also interviewing with Debevoise and Plimpton at the time. This is from the Senate report on the Aguirre affair:

The e-mail was dated September 8, 2005 and addressed to Paul Berger with the subject line, "Debevoise.'' The body of the message read, "Mary Jo [White] just called. I mentioned your interest.''

So Berger was passing notes in class to Mary Jo White about wanting to work for Morgan Stanley's law firm while he was in the middle of quashing an investigation into a major insider trading case involving the C.E.O. of the bank. After the case dies, Berger later gets the multimillion-dollar posting and the circle is closed.

This whole episode highlights everything that's wrong with modern Wall Street. First of all, everybody's buddies with each other – cops and robbers, no adversarial system at all. As Bill Murray would say, it's dogs and cats, living together.

Here, a line investigator gets a good lead, it's quickly taken out of his hands and the whole thing is negotiated at 50,000 feet by friends and former co-workers of the top regulators now working at hotshot firms.

If Barack Obama wanted to send a signal that he's getting tougher on Wall Street, he sure picked a funny way to do it, nominating the woman who helped John Mack get off on the slam-dunkiest insider trading case ever to cross an SEC investigator's desk.

When I contacted Gary today, his take on it was simple. "Obama is not going to clean up financial corruption," he said, "by pinning a sheriff's badge on Wall Street's protector-in-chief."

Editor's Note: Some people I trust have written in to complain that the Mack incident doesn't encompass White's whole career, and that the mere fact that Obama chose an ex-prosecutor with a record of high-level convictions is a good sign. That may be, and the choice of White may very well represent a departure from the last four years. But White's more recent record -- dating back quite a few years now -- has been as a lawyer earning a huge paycheck representing big banks.

Earlier today, I was talking to a former hedge fund manager, a guy who’s been around on Wall Street but is out of the game now. His point about White is simple and it makes a lot of sense. She may very well at one time have been a tough prosecutor. But she dropped out and made the move a lot of regulators make – leaving government to make bucketloads of money working for the people she used to police. “That move, being a tough prosecutor, then going to work defending scumbags, you can only make that move once,” was his point. “You can’t go back again, you know what I mean?”

Think about it: how do you go back and sit in S.E.C.’s top spot after all of those years earning millions as a partner for a firm that represented Morgan Stanley, Bank of America, Goldman Sachs, Deutsche, Chase, and AIG, among others? Think that fact that his firm has retained her firm has anything to do with Jamie Dimon coming out and saying that White is the "perfect choice" to run the S.E.C.? Think of all the things she knows but can’t act upon. Could she really turn around and target Morgan Stanley after being their lawyer for all those years?

Irrespective of the Mack incident, which incidentally really was about as bad as it gets in terms of "regulatory capture," America’s top financial cop should be someone who doesn’t owe his or her nest egg to the world’s biggest banks.