Cambridge’s Helmand restaurant is owned by Mahmood Karzai, brother of Afghan president Hamid Karzai and reportedly under investigation by U.S. prosecutors. (Photo: Ed Kopp)

Mahmood Karzai, owner of The Helmand restaurant in East Cambridge and brother of Afghan President Hamid Karzai, is under investigation on corruption charges from U.S. prosecutors, The Wall Street Journal reported Monday.

“I’m just a businessman,” Mahmood Karzai is quoted as telling the Journal in a story saying prosecutors are considering tax evasion, racketeering or extortion charges. (There are separate charges being mulled against another Karzai brother, Ahmed Wali Karzai, who is in Afghanistan.)

Prosecutors are reportedly curious how Mahmood Karzai built his wealth as a restaurateur to his current worth of $12 million.

The Journal and Agence France-Presse do not note his reputed ownership of Helmand and Helmand Palace restaurants in San Francisco and Baltimore — three of the roughly 80 Afghan restaurants in the United States, according to afghanrestaurants.com. Cambridge’s Annapurna restaurant serves Afghan food as well as Nepalese, Indian and Tibetan cuisine, but the non-Karzai restaurant outside Porter Square began as Buzkashi, an all-Afghan restaurant named after the country’s national sport.