North Korea has denied any involvement in the attack, despite the presentation of forensic evidence on Thursday — including parts of the torpedo found in the wreckage — that experts from three countries said established that the torpedo was launched from a North Korean submarine.

Although the American officials who spoke about the intelligence assessment would not reveal much about what led them to conclude that Mr. Kim was directly involved, one factor appeared to be intelligence that he appeared on April 25, the anniversary of the founding of the Korean People’s Army, with a military unit that intelligence agencies believe to have been responsible for the attack.

Mr. Kim used the event to praise the group, Unit 586, the officials said, and around that time a fourth star appears to have been given to Gen. Kim Myong-guk, who officials believe may have played a crucial role in executing the attack. General Kim is believed to have been demoted to a three-star general last year, perhaps in response to the humiliation that took place after a North Korean ship ventured into South Korean waters. The North Korean ship was all but destroyed, and some analysts believe the attack on the Cheonan, which was in South Korean waters, was planned as retribution.

“Nobody is going to take overt credit for the sinking,” said Jonathan Pollack, a professor at the Naval War College and an expert on North Korea’s military. “But Kim’s visit to this unit has all the hallmarks of congratulating them for a job well done.”

The senior American officials, who spoke on the condition of anonymity because the intelligence assessment is classified, said they ruled out the idea that General Kim or another military officer decided on his own to attack, but they did not explain how they reached that conclusion.

Victor Cha, a North Korea expert at the Center for Strategic and International Studies in Washington and a former official in the National Security Council during President George W. Bush’s second term in office, noted that when Mr. Kim was on the rise three decades ago, “there were similar incidents designed to build his credibility” as a leader.

The Cheonan episode has posed some difficult choices for the Obama administration at a time when its national security team is preoccupied with Afghanistan, Pakistan and Iran.