The Penglai 19-3 oil leak, which polluted 6,200 square kilometers, an area roughly nine times the size of Singapore, was caused by illegal operation, said the State Oceanic Administration in a statement on Friday. [learn more through our full coverage: Oil Spills in North China Sea]

Results of an official investigation show ConocoPhillips, the operator, did not follow its developing plan for the oilfield's two platforms in Bohai Bay and affected the stability between layers under the seabed, which triggered the spill.

The United States energy giant was also late in responding to the leak, said the State Oceanic Administration in a statement on Friday.

The findings contradict the oil company's earlier explanation that a leak detected on June 4 was caused by a natural fault and may have been activated by reservoir pressure.

In an e-mail response to the statement, company spokesman John McLemore said: "ConocoPhillips sincerely regrets these unfortunate incidents. We have fully cooperated throughout the extensive and thorough investigation, and have learned very important lessons."

He said the company is implementing additional, customized enhancements to improve the safety of its operations. "We believe these lessons will help us in China and around the world," he added.

China National Offshore Oil Corp (CNOOC), a partner with ConocoPhillips in the oilfield project, said in an e-mail statement that it will assist the US oil firm to deal with the subsequent issues in a proper manner.

The State Oceanic Administration did not reveal when it will file a lawsuit against ConocoPhilips.

The probe into the leak, launched in August, was conducted by officials from the ministries of environmental protection, transport, agriculture, land and resources, and the State administrations of work safety, oceans and energy resources.

Lawyers and environmentalists applauded the release of the investigation and said the result will accelerate the legal procedure.

"The investigation shows ConocoPhillips' illegal operation directly caused the incident and it should pay for the damages, environmentally and economically," said Wang Yamin, an associate professor at Shandong University's Marine College.

He said the findings can now be used as evidence by victims of the spill.

Ma Jun, director of the Institute of Public and Environmental Affairs, agreed and added that man-made errors and natural faults are totally different explanations for the incident.

"We can be sure ConocoPhillips not only hid the incident from the public, but also lied about the reason," he added. Although the leak began on June 4, it was not revealed to the public until Southern Weekly published details on June 30. ConocoPhillips suspended production on the two leaking platforms at Penglai 19-3 on Sept 2 on the orders of the State Oceanic Administration.

CNOOC said on Oct 24 that the leaks had been sealed by reservoir pressure reduction and a series of technological measures. However, the latest daily monitoring report released on Thursday by the ocean watchdog's North China Sea branch said a 20,000-square-meter oil sheen had been detected and cleaned near platform C.

Although progress has been made, some lawyers and affected fishermen are hoping authorities will make more efforts to investigate the link between the spill and economic losses.

Wang Haijun, a Beijing lawyer who represented 11 environmental groups in filing a lawsuit against ConocoPhillips and CNOOC, said the key in getting compensation for the victims is to prove the spill directly caused the death of scallops, shrimps and sea cucumbers.

"With the investigation report, the affected fishermen have comprehensive evidence," he said, "but more needs to be done."

Yang Jizhen, chairman of the Laoting Fisheries Association in Hebei province, said his lawsuit against ConocoPhillips was rejected because he does not have enough evidence.

"We are facing economic losses of more than 300 million yuan ($47 million), but the court just doesn't accept our sample test results," Yang said. "All we can do is wait for government action."