Editor's Note: Since this story was published, an Australian ship heard a third "pulse" signal, and several more ships are rushing to the area to improve the search. Read our ongoing coverage of the Malaysian Airlines Flight 370 search here.
PERTH, Australia - The head of the multinational search for the missing Malaysia Airlines Flight 370 said Sunday that two electronic pulses reportedly picked up by a Chinese ship were an encouraging sign but stressed they are not yet verified.
Retired Air Chief Marshal Angus Houston told reporters in Perth that two naval ships with high-technology equipment were being sent to the area where the signals were reported to try to confirm or rule out whether they were from the missing plane's flight recorders.
"This is an important and encouraging lead," Houston said of reported acoustic detections from the Haixun 01, but urged caution in drawing any conclusions before the signals were verified.
He said that the Australian vessel Ocean Shield also was investigating a separate acoustic detection.
Officials said that up to 12 military and civilian planes and 13 ships would take part in Sunday's search, which will focus on three areas totaling 83,400 square miles.
The Haixun 01 first reported hearing a "pulse signal" Saturday in Indian Ocean waters with the same frequency emitted by the plane's data recorders. China's official Xinhua News Agency said a black box detector deployed by the Haixun 01 picked up a signal at 37.5 kilohertz (cycles per second).
On Sunday morning, Chinese officials told the Joint Agency Coordination Center - the Australian government agency coordinating the search - that Haixun 01 had detected the signal for a second time within 1.2 miles of the first occurrence, said Houston, the agency's chief coordinator.
With the batteries in the black boxes' locator beacons due to run out any day, crews are in a desperate race against the clock.
The two naval ships -- the Ocean Shield and the British Navy's HMS Echo - are to join the Chinese ship as expeditiously as possible, Houston said on Sunday.
On Friday for the first time, the Ocean Shield and HMS Echo launched an underwater search trying to pick up a signal from the black box flight recorders on the Beijing-bound plane before they are expected to fall silent. The batteries last only about 30 days, which would be Monday.
The search for the Boeing 777 -- plagued by confusion, and agonizing to relatives of the 239 passengers -- has frustrated investigators and left many wondering how long it can go on.
The two ships began probing the ocean along a 150-mile route that investigators hope is close to where Malaysia Airlines Flight 370 went down. The Ocean Shield is towing a U.S. Navy device that can detect signals or pings from the flight data and cockpit voice recorders, commonly known as the black boxes.
The U.S. Navy's towed pinger locator can pick up signals to a depth of 20,000 feet, and so should be able to hear the plane's data recorders even if they are at the deepest part of the search zone, about 19,000 feet.
But no wreckage from the plane has been found, so officials cannot even be sure they are looking in the right location. The 84,000-square-mile search area, about 1,100 miles northwest of Perth, was already shifted almost 700 miles to the north after investigators decided that the plane was traveling faster than originally thought.
Plus, the pinger locator -- consisting of a 30-inch cylindrical microphone attached to about 20,000 feet of cable -- must be dragged slowly through the water at just 1 to 5 knots (or 1 to 6 mph) in a grid pattern. And false alerts can come from animals such as whales, or interference from shipping noise.
Australian officials continue to refine the area where the plane entered the water based analysis of satellite communication and the aircraft's performance. On Sunday, Houston said that as a result of a correction to the satellite data that the southern component of the search area had a higher priority.
The Malaysia Airlines jet left Kuala Lumpur on Saturday, March 8, at 12:41 a.m. headed for Beijing. But investigators believe someone re-programmed the plane's flight management system, and two minutes after the last conversation between air traffic controllers in Malaysia and the cockpit, the plane's transponder was turned off. The plane went dark on civilian radar, and then made a left turn back toward Malaysia.
Sources have said it followed an established aviation corridor over several navigational "waypoints."
The Malaysian military tracked an unidentified object now believed to have been Flight 370 on its radar traveling west towards the Strait of Malacca. At 2:15 a.m., it disappeared from the military radar, about 200 miles northwest of Penang.
Investigators say the plane's antenna signaled to a satellite multiple times over the next several hours, with the last signal recorded at 8:11 a.m., about the time the plane would have run out of fuel.
Experts says the search in the southern Indian Ocean might have been easier had the plane been outfitted with so-called "deployable black box" technology, essentially flight recorders that eject and float when a plane crashes.
The U.S. Transportation Safety Administration tested the technology, employed on U.S. Navy jets such as the F/A-18 for more than two decades, and found that it would enhance safety on commercial aircraft. But three years after the study, no U.S. commercial airline has installed the technology. The price tag per plane is about $60,000.