PERTH, Australia (AP) -- A robotic submarine hunting for the missing Malaysian jet aborted its first mission after only six hours, surfacing with no new clues when it exceeded its maximum depth along the floor of the Indian Ocean, officials said Tuesday.
Search crews sent the U.S. Navy's Bluefin 21 into the depths Monday to begin scouring the seabed for the missing Malaysia Airlines Boeing 777 after failing for six days to detect any new signals believed to be coming from its black boxes.
But the 16-hour mission was cut short when the unmanned sub, which is programmed to hover 30 meters (100 feet) above the seabed, entered a patch that was deeper than its maximum depth of 4,500 meters (15,000 feet), the search coordination center and the U.S. Navy said.
A built-in safety feature returned the Bluefin to the surface and it was not damaged, they said.
The data collected by the sub was later analyzed and no sign of the missing plane was found, the U.S. Navy said. Crews were shifting the Bluefin's search area away from the deepest water and were hoping to send it back on another mission later Tuesday.
Search authorities had known the primary search area for Flight 370 was near the limit of the Bluefin's dive capabilities. Deeper-diving submersibles have been evaluated, but none is yet available to help.
A safety margin would have been included in the Bluefin's program to protect the device from harm if it went a bit deeper than its 4,500-meter limit, said Stefan Williams, a professor of marine robotics at the University of Sydney.
"Maybe some areas where they are doing the survey are a little bit deeper than they are expecting," he said. "They may not have very reliable prior data for the area."
Meanwhile, officials were investigating an oil slick about 5,500 meters (3.4 miles) from the area where the last underwater sounds were detected.
Crews collected an oil sample and sent it back to Perth in western Australia for analysis, a process that will take several days, said Angus Houston, the head of the joint agency coordinating the search off Australia's west coast.
He said it does not appear to be from any of the ships in the area, but cautioned against jumping to conclusions about its source.
The Bluefin can create a three-dimensional sonar map of any debris on the ocean floor. But the search is more challenging in this area because the seabed is covered in silt that could potentially cover part of the plane.
"What they're going to have to be looking for is contrast between hard objects, like bits of a fuselage, and that silty bottom," Williams said. "With the types of sonars they are using, if stuff is sitting up on top of the silt, say a wing was there, you could likely see that ... but small items might sink down into the silt and be covered and then it's going to be a lot more challenging."
The search moved below the surface after crews picked up a series of underwater sounds over the past two weeks that were consistent with signals from an aircraft's black boxes, which record flight data and cockpit conversations. The devices emit "pings" so they can be more easily found, but their batteries last only about a month and no sounds have been heard for seven days.
Australian Prime Minister Tony Abbott raised hopes last week when he said authorities were "very confident" the underwater signals were from the black boxes on Flight 370, which disappeared March 8 during a flight from Kuala Lumpur, Malaysia, to Beijing with 239 people on board, mostly Chinese.
Houston said Monday that the signals were a promising lead, but that finding aircraft wreckage in the remote, deep patch of ocean remains extremely difficult.
The submarine is programmed to take 24 hours to complete each mission: two hours to dive to the bottom, 16 hours to search the seafloor, two hours to return to the surface, and four hours to upload the data.
The black boxes could contain the key to unraveling the mystery of what happened to Flight 370. Investigators believe the plane went down in the southern Indian Ocean based on a flight path calculated from its contacts with a satellite and an analysis of its speed and fuel capacity. But they still don't know why.
On Tuesday, Malaysia's defense minister, Hishamuddin Hussein, pledged to reveal the full contents of the black boxes if they are found.
"It's about finding out the truth," he told reporters in Kuala Lumpur. "There is no question of it not being released."
Up to 11 planes and as many ships were scouring a 62,000-square kilometer (24,000-square mile) patch of ocean about 2,200 kilometers (1,400 miles) northwest of Perth on Tuesday, hunting for any floating debris.
The weekslong surface search is expected to end in the next two days. Officials haven't found a single piece of debris confirmed to be from the plane, and Houston said the chances that any would be found have "greatly diminished."
Associated Press writers Kristen Gelineau in Sydney and Rod McGuirk in Canberra, Australia contributed to this report.
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ANAHEIM, Calif. (AP) -- Two convicted sex offenders dutifully checked in with police every month and wore their GPS trackers around the clock - the rules of parole that are designed to tip off authorities if a freed felon backslides.
Yet for at least two months last fall, authorities claim, Franc Cano and Steven Dean Gordon were raping and killing at least four women - and probably a fifth - in the seedy prostitution hangouts of Orange County.
It was data from their GPS trackers - along with cellphone records from the victims and other evidence - that helped investigators link them to the killings, police said.
"That was one of the investigative tools we used to put the case together," Anaheim Police Chief Raul Quezada said at a news conference Monday.
Cano, 27, and Gordon, 45, were arrested by investigators on Friday. Each was charged Monday with four felony counts of special circumstances murder and four felony counts of rape.
If convicted, they could face a minimum sentence of life without parole or the death penalty. They were being held without bail and expected to be arraigned Tuesday.
The men had known each other at least since 2012, when they cut off their GPS trackers and, using fake names, fled to Las Vegas, where they stayed at the Circus Circus Hotel & Casino for two weeks before they were rearrested, according to documents filed in U.S. District Court in Nevada.
While out on parole, police believe the men killed three women in Santa Ana last October and November and an another woman in Anaheim earlier this year. All had histories of prostitution.
Quezada said authorities were confident that there was at least a fifth victim and perhaps more.
Investigators "put a stop to a serial killing that would likely have continued beyond this point," District Attorney Tony Rackauckas said.
The department has contacted other places with missing-persons cases across the country.
Kianna Jackson, 20, of Las Vegas, arrived in Santa Ana the first week of October for a court hearing on four misdemeanor charges of prostitution and loitering to commit prostitution. Her mother said she stopped responding to her text messages soon after she arrived in Santa Ana.
She checked in to a Costa Mesa hotel but never paid the bill nor checked out, and her belongings were found there.
Josephine Monique Vargas, 34, was last seen Oct. 24 after leaving a family birthday party in Santa Ana to go to a store.
Martha Anaya, 28, asked her boyfriend to pick up their 5-year-old daughter so she could work on Nov. 12, then stopped responding to his messages later that night. She had been planning a birthday party for her daughter.
Santa Ana investigators didn't realize that they were looking for murder victims at first, Police Chief Carlos Rojas said.
Instead, police considered them missing persons. Investigators searched a canyon, examined the women's cellphone records, alerted hospitals, put the word out on social media and even checked motels they were known to frequent but without success in finding them.
Then, on March 14, the naked body of Jarrae Nykkole Estepp, 21, was found March 14 on a conveyor belt at an Anaheim trash-sorting plant.
That was the key that broke the case, authorities said.
In the weeks before the discovery, Estepp had become a regular on a strip of Beach Boulevard in Anaheim long known for prostitution.
Estepp had "a similar profile to our victims; we were able to ... move forward," Rojas said.
Investigators planned to search for the bodies of the three Santa Ana victims, he said.
Cano and Gordon each served time after being convicted in separate cases of lewd and lascivious acts with a child under 14.
Gordon was convicted in 1992 and has a 2002 kidnapping conviction, according to the Orange County district attorney's office. Cano's conviction dates to 2008, prosecutors said.
After their Las Vegas escapade, Cano and Gordon pleaded guilty to failure to register as a sex offender. They were ordered to provide DNA samples and have their computers monitored by federal agents, according to the federal documents, which were first obtained by the Los Angeles Times.
The men also checked in with Anaheim police every 30 days, as required, and provided updated photos, fingerprints and addresses, Anaheim police Lt. Bob Dunn said.
In fact, both men checked in earlier this month, Dunn said.
Cano was wearing a state-issued ankle monitor and Gordon was wearing a federal GPS device, he said.
Associated Press writers Anthony McCartney and Daisy Nguyen in Los Angeles and Amy Taxin in Santa Ana contributed to this report.