PERTH, Australia (AP) -- An Australian aircraft Thursday detected what may be the fifth signal coming from a man-made device deep in the Indian Ocean, adding to hopes that searchers will soon pinpoint the object's location and send down a robotic vehicle to confirm if it is a black box from the missing Malaysian jet.
The Australian air force P-3 Orion, which has been dropping sonar buoys into the water near where four earlier sounds were heard, picked up a "possible signal" that may be from a man-made source, said Angus Houston, who is coordinating the search off Australia's west coast.
"The acoustic data will require further analysis overnight," Houston said in a statement.
If confirmed, the signal would add further narrow the hunt for Malaysia Airlines Flight 370, which vanished on March 8 while flying from Kuala Lumpur to Beijing with 239 people aboard.
The Australian ship Ocean Shield picked up two underwater sounds on Tuesday, and two sounds it detected Saturday were determined to be consistent with the pings emitted from a plane's flight recorders, or "black boxes."
The Australian air force has been dropping sonar buoys to better pinpoint the location of the sounds detected by the Ocean Shield in a search zone that is now the size of the city of Los Angeles.
Royal Australian Navy Commodore Peter Leavy said each buoy is dangling a hydrophone listening device about 300 meters (1,000 feet) below the surface. Each buoy transmits its data via radio back to the plane.
The underwater search zone is currently a 1,300-square-kilometer (500-square-mile) patch of the ocean floor, and narrowing the area as much as possible is crucial before an unmanned submarine can be sent to create a sonar map of a potential debris field on the seabed.
The Bluefin 21 sub takes six times longer to cover the same area as the pinger locator being towed by the Ocean Shield, and it would take the vehicle about six weeks to two months to canvass the current underwater search zone. That's why the acoustic equipment is still being used to hone in on a more precise location, U.S. Navy Capt. Mark Matthews said.
The search for floating debris on the ocean surface was narrowed Thursday to its smallest size yet - 57,900 square kilometers (22,300 square miles), or about one-quarter the size it was a few days ago. Fourteen planes and 13 ships were looking for floating debris, about 2,300 kilometers (1,400 miles) northwest of Perth.
A "large number of objects" were spotted on Wednesday, but the few that had been retrieved by search vessels were not believed to be related to the missing plane, the search coordination center said.
Crews hunting for debris on the surface have already looked in the area they were crisscrossing on Thursday, but were moving in tighter patterns, now that the search zone has been narrowed to about a quarter the size it was a few days ago, Houston said.
Houston has expressed optimism about the sounds detected earlier in the week, saying on Wednesday that he was hopeful crews would find the aircraft - or what's left of it - in the "not-too-distant future."
The locator beacons on the black boxes holding the flight data and cockpit voice recorders have a battery life of about a month, and Tuesday marked one month since Flight 370 disappeared. The plane veered off-course for an unknown reason, so the data on the black boxes are essential to finding the plane and solving the mystery. Investigators suspect it went down in the southern Indian Ocean based on a flight path calculated from its contacts with a communications satellite and analysis of its speed and when it would have run out of fuel.
An Australian government briefing document circulated among international agencies involved in the search on Thursday said it was likely that the acoustic pingers would continue to transmit at decreasing strength for up to 10 more days, depending on conditions.
Once there is no hope left of the Ocean Shield's equipment picking up any more sounds, the Bluefin sub will be deployed.
Complicating matters, however, is the depth of the seafloor in the search area. The pings detected earlier are emanating from 4,500 meters (14,763 feet) below the surface - which is the deepest the Bluefin can dive.
"It'll be pretty close to its operating limit. It's got a safety margin of error and if they think it's warranted, then they push it a little bit," said Stefan Williams, a professor of marine robotics at Sydney University.
The search coordination center said it was considering available options in case a deeper diving sub is needed. But Williams suspects if that happens, the search will be delayed while an underwater vehicle rated to 6,000 meters (19,700 feet) is dismantled and air freighted from Europe, the U.S. or Japan.
Williams said colleagues at the Woods Hole Oceanographic Institution in Massachusetts had autonomous and remotely operated underwater vehicles that will dive to 11 kilometers (36,100 feet), although they might not be equipped for such a search.
Underwater vessels rated to 6,500 meters (21,300 feet) could search the seabed of more than 90 percent of the world's oceans, Williams said.
"There's not that much of it deeper than 6 1/2 kilometers," he said.
Williams said it was unlikely that the wreck had fallen into the narrow Diamantina trench, which is about 5,800 meters (19,000 feet) deep, since sounds emanating from that depth would probably not have been detected by the pinger locator.
Gelineau reported from Sydney. Associated Press Writer Rod McGuirk in Canberra, Australia, contributed to this report.
MURRYSVILLE, Pa. (AP) -- It was just before the start of class and the hallways were packed as usual with students at their lockers or chatting with friends.
Nate Moore was walking to homeroom, book in hand, when a classmate he knew to be quiet and unassuming tackled a freshman boy a few feet in front of him. Moore thought it was the start of a fistfight and went to break it up.
But 16-year-old Alex Hribal wasn't throwing punches - he was stabbing his victim in the belly, Moore said. The suspect got up and slashed Moore's face, then took off down the hall, where authorities said he stabbed and slashed other students in an attack that injured 21 students and a security guard - and might have been even worse but for the "heroes" who Pennsylvania's governor said helped prevent further injury or loss of life.
An assistant principal tackled and subdued Hribal, who was charged Wednesday night with four counts of attempted homicide and 21 counts of aggravated assault and jailed without bail. Authorities said he would be prosecuted as an adult.
The suspect's motive remained a mystery.
"He wasn't saying anything," Moore recalled hours later. "He didn't have any anger on his face. It was just a blank expression."
At a brief hearing Wednesday night, District Attorney John Peck said that after he was taken into custody, Hribal made comments suggesting he wanted to die. Defense attorney Patrick Thomassey described him as a good student who got along with others, and asked for a psychiatric examination.
At least five students were critically wounded in the attack, including a boy who was on a ventilator after a knife pierced his liver, missing his heart and aorta by only millimeters, doctors said. He had an additional surgery overnight, they said.
The rampage comes after decades in which U.S. schools have focused their emergency preparedness on mass shootings, not stabbings.
While knife attacks at schools are not unusual, they're most often limited to a single victim, said Mo Canady, executive director of the National Association of School Resource Officers.
Nevertheless, there have been at least two major stabbing attacks at U.S. schools over the past year, the first at a community college in Texas last April that wounded at least 14 people, and another, also in Texas, that killed a 17-year-old student and injured three others at a high school last September.
The attack in Pittsburgh unfolded shortly after 7 a.m. Wednesday, a few minutes before the start of classes at 1,200-student Franklin Regional High School, in an upper-middle-class area 15 miles east of Pittsburgh. By Thursday morning, the school was no longer being treated as a crime scene, according to police and school officials, who said they expected it to reopen Monday.
Mia Meixner, 16, said the freshman boy who was tackled tried to fight back, then, when his assailant got off him, stood up and lifted his shirt to reveal a midsection covered in blood.
"He had his shirt pulled up and he was screaming, `Help! Help!'" said another witness, Michael Float, 18. "He had a stab wound right at the top right of his stomach, blood pouring down."
As students rushed to the boy's aid, the attacker slashed Moore before taking off around a bend.
"It was really fast. It felt like he hit me with a wet rag because I felt the blood splash on my face. It spurted up on my forehead," said Moore, whose gashed right cheek required 11 stitches.
The boy ran down about 200 feet of hallway, slashing and stabbing other students with kitchen knives about 8 to 10 inches long, police said. The assault touched off a "stampede of kids" yelling, "Run! Get out of here! Someone has a knife!" according to Meixner.
Assistant Principal Sam King heard the commotion and found a chaotic scene in the blood-soaked hall.
"I've been stabbed," he heard a student say.
King then saw Hribal stab a security guard, who leaned against the wall, bleeding from his stomach, according to a police affidavit. King tackled Hribal and kept him on the floor until a school police officer handcuffed him.
The rampage lasted about five minutes.
"There are a number of heroes in this day. Many of them are students," Gov. Tom Corbett said in a visit to the stricken town. "Students who stayed with their friends and didn't leave their friends."
He also commended cafeteria workers, teachers and teacher's aides who put themselves at risk to help others.
Looking for a motive, Murrysville Police Chief Thomas Seefeld said investigators were checking reports of a threatening phone call between Hribal and another student the night before. He didn't say whether the suspect received or made the call.
The FBI went to the boy's house, and local media reports said agents removed at least one computer along with other items.
Meixner and Moore called the attacker a shy and quiet boy who largely kept to himself, but they said he was not an outcast and they saw no indication before the attack that he might be violent.
"He was never mean to anyone, and I never saw people be mean to him," Meixner said. "I never saw him with a particular group of friends."
During the attack, the boy had a "blank look," she said. "He was just kind of looking like he always does, not smiling, not scowling or frowning."
Associated Press writers Michael Rubinkam in northeastern Pennsylvania, Joe Mandak in Pittsburgh and JoAnn Loviglio in Philadelphia contributed to this report.