U.S. regulators probing the May flash crash are focusing on a trading practice known as "quote stuffing", in which large numbers of rapid-fire orders to buy or sell stocks are placed and canceled almost immediately.

CFTC commissioner Scott O'Malia told Reuters on Thursday that the futures regulator was reviewing data from Nanex LLC, a trade database developer that issued a study suggesting that computer algorithms used quote stuffing to gain an edge during the May 6 crash.

The U.S. Securities and Exchange Commission, which is investigating the crash jointly with the Commodity Futures Trading Commission, is looking at quote stuffing and something called "sub-penny pricing", a person familiar with the flash crash probe said.

The Nanex study uses market graphics and playful names to illustrate quote stuffing, arguing that high-frequency trading firms do this to flood the marketplace with bogus orders to distract rival trading firms.

Investors could make trades under the false impression that those orders were legitimate, only to see liquidity disappear and the market move against them when the orders are canceled -- all in the blink of an eye.

The Justice Department's Public Integrity Section has a storied 34-year history of pursuing corruption in government and safeguarding the public trust.

That trust was breached, however, when some of the unit's prosecutors failed to turn over evidence favorable to the defense in their high-profile criminal trial of Sen. Ted Stevens, R-Alaska, who died earlier this month in a plane crash.

Now Jack Smith, a 41-year-old prosecutor with a love for courtroom work and an impressive record, has been brought in to restore the elite unit's credibility.

Before Stevens, Public Integrity's renown was built on large successes — like the prosecution of the Jack Abramoff lobbying scandal and convictions of federal and state judges, members of Congress and state legislators, military officers, federal lawmen and bureaucrats and their state counterparts over the years.

But its stumble — not disclosing exculpatory evidence as Supreme Court precedent requires — was equally large. It was so serious that Attorney General Eric Holder, one of Public Integrity's distinguished alums, stepped in and asked a federal judge to throw out Stevens' convictions.

At the time of the Stevens debacle, Smith was overseeing all investigations for the international war crimes office at The Hague in the Netherlands. He'd read about the Stevens case. Offered the chance to take over Public Integrity, he couldn't stay away.

Democratic Party Chairman Tim Kaine is defending first lady Michelle Obama's vacation trip to Spain, saying critics of her travels are trying to politicize the issue.

Kaine tells NBC's "Today" show he thinks "it's wrong" to talk critically about her trips. Critics contend they send a poor message at a time when many Americans are out of work.

Kaine said, "She's a mom." He said this was an opportunity for her to take nine-year-old daughter Sasha to a part of the world she hadn't seen before.

Kaine said President Barack Obama and Michelle Obama are "focused on being good parents." Mrs. Obama returned with Sasha to the White House late Sunday. Her trip occurred as Obama was celebrating his 49th birthday and their other daughter, 12-year-old Malia, was away at summer camp.