SEOUL, South Korea – The way U.S. officials see it, there’s little mystery behind the most notorious shipwreck in recent Korean history.

Secretary of State Hillary Rodham Clinton calls the evidence “overwhelming” that the Cheonan, a South Korean warship that sank in March, was hit by a North Korean torpedo.

But challenges to the official version of events are coming from an unlikely place — within South Korea itself.

Armed with dossiers of their own scientific studies and bolstered by conspiracy theories, critics dispute the findings announced May 20 by South Korean President Lee Myung-bak, which pointed a finger at Pyongyang, the North Korean seat of government.

They also question why Lee made the announcement nearly two months after the ship’s sinking, on the very day campaigning opened for fiercely contested local elections. Many accuse the conservative leader of using the deaths of 46 sailors to stir up anti-communist sentiment and sway the vote.

The critics, mostly but not all from the opposition, say it is unlikely that the impoverished North Korean regime could have pulled off a perfectly executed hit against a superior military power, sneaking a submarine into the area and slipping away without detection. They also wonder whether the evidence of a torpedo attack was misinterpreted, or even fabricated.

“I couldn’t find the slightest sign of an explosion,” said Shin Sang-chul, a former shipbuilding executive-turned-investigative journalist. “The sailors drowned to death. Their bodies were clean. We didn’t even find dead fish in the sea.”

The doubts about the Cheonan have embarrassed the United States, which is beginning joint military exercises Sunday in a show of unity against North Korean aggression. On Friday, an angry North Korea warned that “there will be a physical response” to the maneuvers.

Two South Korean-born U.S. academics have joined the chorus of skepticism, holding a news conference this month in Tokyo to voice their suspicions about the “smoking gun” — a piece of torpedo propeller with a handwritten mark in blue ink reading “No. 1” in Korean.

“You could put that mark on an iPhone and claim it was manufactured in North Korea,” scoffed one of the academics, Seunghun Lee, a professor of physics at the University of Virginia.