Three weeks after a flood of heavy crude oil fouled scores of miles of this immense northern city's beaches and rocky coastline, a remarkable -- some would say heroic -- cleanup effort has scrubbed away most traces of the spill. Whether Dalian's government can eradicate damning accounts of a major industrial accident and a narrowly averted catastrophe that could have killed thousands, on the other hand, remains in considerable doubt. Since July 16, when an explosion and fire ruptured a pipe linking a berthed oil tanker to an onshore storage facility at the PetroChina port, officials here have strained to minimize the scope of the damage. Their version of events, released shortly after the spill, states that a broken pipe leaked 1,500 tons, or about 11,000 barrels, of crude into the bay off Dalian's northeastern coast. It states that the spill occurred after workers unloading oil from the berthed ship accidentally set off an explosion and fire and that the oil was quickly contained before it could enter international waters. That remains the official story this week. But an investigation by the environmental advocacy group Greenpeace and accounts by two experts with authoritative knowledge of what happened make a compelling case for an alternative account. The Greenpeace report, issued last week, said that as the fire from the explosion spread, large quantities of oil were deliberately released from onshore tanks to avert devastating damage to a tank at the storage facility that was filled with dimethylbenzene, a flammable and poisonous gas used to make aviation fuel and solvents. Those experts, who asked for anonymity out of fear of government retaliation, said emergency workers deliberately opened release valves on one huge oil tank, fearing the fire could cause it to explode and crack open the tank of toxic gas. The emptied tank most likely was filled with 50,000 tons, or between 315,000 and 365,000 barrels, of crude. Greenpeace experts said much of that could have burned off in the fire that night. But other experts said in interviews that they were skeptical that much of the released oil had burned off because heavy crude of the sort involved in the Dalian spill is less flammable than lighter oil. A satellite photograph taken two days after the Dalian spill and published on the environmental website skytruth.org suggests that the oil had spread far into the sea from the spill site and was swept along the coastline. China National Petroleum Corp., the parent company of PetroChina, states on its website that the Dalian accident released 'a small amount' of oil and sewage water into the bay. The company's officials did not return a request for comment on the accounts of a much larger accident. 