When Afghan security forces found an enormous cache of heroin hidden beneath concrete blocks in a tractor-trailer outside Kandahar in 2004, the local Afghan commander quickly impounded the truck and notified his boss. Before long, the commander, Habibullah Jan, received a telephone call from Ahmed Wali Karzai, the brother of President Hamid Karzai, asking him to release the vehicle and the drugs, Mr. Jan later told American investigators, according to notes from the debriefing obtained by The New York Times. He said he complied after getting a phone call from an aide to President Karzai directing him to release the truck. Two years later, American and Afghan counternarcotics forces stopped another truck, this time near Kabul, finding more than 110 pounds of heroin. Soon after the seizure, US investigators told other American officials that they had discovered links between the drug shipment and a bodyguard believed to be an intermediary for Ahmed Wali Karzai, according to a participant in the briefing. The assertions about the involvement of the president's brother in the incidents were never investigated, according to American and Afghan officials, even though allegations that he has benefited from narcotics trafficking have circulated widely in Afghanistan. Both President Karzai and Ahmed Wali Karzai, now the chief of the Kandahar Provincial Council, the governing body for the region that includes Afghanistan's second largest city, dismiss the allegations as politically motivated attacks by longtime foes. "I am not a drug dealer, I never was and I never will be," the president's brother said in a recent phone interview. "I am a victim of vicious politics."  But the assertions about him have deeply worried top American officials in Kabul and in Washington. The United States officials fear that perceptions that the Afghan president might be protecting his brother are damaging his credibility and undermining efforts by the United States to buttress his government, which has been under siege from rivals and a Taliban insurgency fueled by drug money, several senior Bush administration officials said. Their concerns have intensified as American troops have been deployed to the country in growing numbers. "What appears to be a fairly common Afghan public perception of corruption inside their government is a tremendously corrosive element working against establishing long-term confidence in that government " a very serious matter," said Lt. Gen. David W. Barno, who was commander of coalition military forces in Afghanistan from 2003 to 2005 and is now retired. "That could be problematic strategically for the United States." The White House says it believes that Ahmed Wali Karzai is involved in drug trafficking, and American officials have repeatedly warned President Karzai that his brother is a political liability, two senior Bush administration officials said in interviews last week.