(USATODAY.com) - The quest to solve the mystery of Malaysia Airlines Flight 370 took a step backward Thursday when Australian authorities determined that a small oil slick discovered Sunday in the prime search area was not from the missing Boeing 777.
"Preliminary analysis of the sample collected by ADV (Australian Defense Vessel) Ocean Shield has confirmed that it is not aircraft engine oil or hydraulic fluid," Australia's Joint Agency Coordination Center said in a statement.
The center also said that the operators of the robotic submarine Bluefin-21 have determined that there is a "small but acceptable level of risk" in operating the Bluefin-21 at depths approaching 3 miles, which would allow it to search the ocean floor in the deepest area of the target area.
The sub's search should take less than the initial estimate of up to two months because the targeted area has been trimmed, the center added. Bluefin-21 completed its first full 16-hour mission Thursday.
The JACC is leading the effort to find the jet, which disappeared with 239 people aboard shortly after takeoff from Kuala Lumpur, Malaysia, en route to Beijing on March 8.
In addition to the U.S. Navy's Bluefin-21 submarine, up to 12 planes and 11 ships continued to search for the plane Thursday. The planes and ships were covering a search area totaling about 15,579 square miles. The center of the area lies 1,348 miles northwest of Perth, Australia, and the Bluefin-21 has covered 35 square miles so far.
Australian search officials had said they picked up "possible signals" from a man-made source more than a week ago and those signals have now stopped. The Australian air force P-3 Orion, which has been dropping sonar buoys into the water near where the sounds were picked up four times, have not officially verified the pings as coming from Malaysia Airlines Flight 370.
The two data recorders, also known as black boxes, emit signals via battery-power. But the battery life is only about 30 days.
Also Thursday, Malaysia's defense minister, Hishamuddin Hussein, confirmed that the search would continue through the Easter weekend, though he acknowledged that officials would have to rethink their strategy at some point if nothing is found.
"There will come a time when we need to regroup and reconsider, but in any event, the search will always continue. It's just a matter of approach," he said at a news conference.
Meanwhile, the families of Chinese passengers who were on the Malaysia Airlines plane continued to lash out at Malaysian authorities for not telling them what is going on.
"You are all bloody liars, and you are lying to us again now," one relative shouted at a teleconference in Beijing with high-level Malaysian government and airline officials in Kuala Lumpur. "You have absolutely no shame."