(CNN) -- Was he ill? Was he asking for Chinese aid? Or was it a straight-up diplomatic snub?
North Korean leader Kim Jong Il reportedly left on an unexpected trip to China, leaving behind former President Jimmy Carter and a dizzying trail of speculation over his timing.
Carter's trip has now been extended by a day, said Han Park, a University of the Georgia professor who helped arrange both of Carter's trips to the Communist state, suggesting the two men still plan to shake hands.
Park, who has been to North Korea 52 times, said he believes the real reason behind Carter's trip was to meet with Kim and that the Korean leader would return to Pyongyang to make that happen.
But Kim's apparent sudden journey by train late Wednesday to northeastern China took observers by surprise.
"Right now, it's all very curious," said Jim Walsh of the Massachusetts Institute of Technology's Security Studies Program. "Do I think this is unusual? Yes I do."
Not meeting with Carter at all would be a definite insult to the former U.S. president, who visited North Korea in 1994 and met with Kim's father, then-leader Kim Il Sung. That trip helped defuse the first nuclear crisis.
"If he left without meeting President Carter, then, yes, that would be perceived as an affront or that he had higher priorities," said Scott Snyder, an adjunct senior fellow for Korea studies at the Council on Foreign Relations.
Snyder said the White House distanced itself from Carter's trip, going out of their way to ensure that it was not seen as an official visit. But no one expected the North Koreans to withdraw from talks with the former leader of the free world -- a man who can help bestow credibility on the rogue state.
In the absence of direct talks, meetings with Carter would also enable North Korea to voice to the United States its demands for security assurances in exchange for abandoning its nuclear program.
It was unclear Thursday whether Kim had already met with Carter or would be returning soon to do so. Or would not be shaking hands at all.
"We can sit back and engage in all sorts of speculation," Walsh said. "They all have equal credibility at this point."
Park said he believed reports that Kim took his son and heir apparent Kim Jong Un to the school in northeastern China that his father attended as a step toward the son's ascension to power.
Walsh, however, said the request for Chinese aid seemed most plausible. If Kim were sick, he probably would have summoned doctors to him.
Kim's special train crossed into China late Wednesday toward the northeastern Chinese border city of Jian, an official at South Korea's presidential office said. Neither Beijing nor Pyongyang confirmed Kim's whereabouts though it is standard practice to keep mum until Kim is back home.
"We detected Kim Jong Il's train heading towards China, over the border, at midnight Wednesday," a source at South Korea's presidential office said. "We assume that Kim is aboard the train, and are trying to find out his destination and the purpose of his visit."
Carter arrived in Pyongyang Wednesday to negotiate the release of U.S. citizen Aijilon Gomes, believed to be a Christian activist, who is currently imprisoned in the country after entering it illegally in January.
It is widely believed that Carter will succeed in his humanitarian mission and return home with Gomes.
Journalist Andrew Salmon contributed to this report.