The Red Cross announced the investigation Thursday; U.S. Attorney Jim Letten in New Orleans confirmed it Friday but declined to comment further. Earlier, the Red Cross and state Attorney General Charles Foti announced they were conducting their own probes.
"The Red Cross takes these allegations very seriously and is committed to being the best stewards of the donated dollar," Red Cross President and CEO Jack McGuire said in a statement dated Thursday and released in Washington.
Sen. Charles Grassley, R-Iowa, has said volunteers may have committed criminal fraud. The accusations include improperly diverting relief supplies and violating Red Cross rules by using felons as volunteers in the disaster area.
The Red Cross had 235,000 volunteers working in the Katrina disaster area, nearly six times the previous peak of 40,000.
The charity said last week that a catastrophe as large as Katrina, which flooded 80 percent of New Orleans and killed more than 1,300 people in Louisiana and Mississippi, leads to "departures from standard procedures" but said it would not tolerate illegal activity.
In Illinois, meanwhile, a 30-year-old woman was sentenced to four years in prison for posing as a Katrina victim and receiving Red Cross relief money.
A Macon County judge sentenced Ockisha Turner on Thursday and ordered her to pay $1,565 in restitution after she pleaded guilty to burglary of the charity's Mid-Illinois chapter.