Rio de Janeiro ? Signals from the black boxes of Air France Flight 447 are fading, weakening along with hopes of resolving what experts are calling one of history’s most challenging plane crash investigations.

Emergency beacons attached to cockpit voice and data recorders are built to emit strong “pings” for 30 days after a crash before fading away, though experts said they could continue for as long as 45 days.

Today marks Day 30 since the plane dropped out of the sky with 228 people on board in a remote area of the Atlantic far off Brazil’s northeastern coast and from radar coverage. A burst of automated messages emitted by the plane before it fell gave rescuers only a vague location to begin their search.

“Without that starting point, the ‘needle in the haystack’ analogy would look like an easy assignment compared to this,” said Peter Goelz, a former managing director of the U.S. National Transportation Safety Board. “This is the most difficult accident in terms of recovery operations that I’ve ever seen.”

Those hunting for the two black boxes said the search will continue. On Tuesday, Martine del Bono, spokeswoman for the French air accident agency leading the investigation, said it “is continuing the search” as long as there is a “reasonable” chance of locating the black boxes. She gave no final deadline.

U.S. Air Force Col. Willie Berges, the Brazil-based commander of the American military forces supporting the effort, has said searchers are likely to keep looking for 12 to 15 days beyond the crash’s 30-day mark. The Americans are operating two U.S. Navy pinger locators that are being towed by French-contracted ships. A French nuclear submarine is scouring a search area with a radius of 50 miles in the area where the plane is thought to have crashed.

The logistics of recovering debris and remains from the Air France flight are complicated by its disappearance 930 miles off Brazil’s mainland. Investigators should have an easier time recovering debris and clues in the crash of a Yemeni Airbus 310 with 153 people on board that went down Tuesday just nine miles north of the Indian Ocean island-nation of Comoros.

The black boxes emit an electronic tapping sound that can be heard up to 1.25 miles away. While searchers for the Air France plane have detected some noise in the deep ocean, they have heard nothing from the flight recorders.

The Airbus A330 jet went down in the middle of the Atlantic shortly after midnight June 1. The crash date had been reported as May 31, as it was 11:14 p.m. on Brazil’s mainland when the plane sent its last automated messages. But as searchers found debris and those messages were made public, it was clear the plane had crossed into a new time zone — and a new day — before it went into the ocean.

Without the crucial evidence the black boxes contain, investigators may never be able to determine definitively why the jet fell — despite the recovery of a substantial amount of wreckage and the remains of 51 people.

“The most you can do is a detailed forensic analysis of what affected the recovered items,” Goelz said. “That may or may not give you a picture of what went on. But it isn’t going to go to the cause of the accident, it will go to what happened after the event occurred.”

With the recorders still missing, investigators are focusing on the automated messages sent by the plane minutes before it lost contact. One indicates the plane was receiving incorrect speed information from external monitoring instruments, which could destabilize the plane’s control systems. Experts have suggested those external instruments might have iced over. Air France has now replaced the monitors, called Pitot tubes, on all its Airbus A330 and A340 aircraft.

But the mystery of what really caused the crash continues, leaving aviation safety experts unclear about what needs to be changed to stop a similar catastrophe in the future.

“Any time you have an accident that remains a question mark, it is a problem for the whole aviation community,” Goelz said. “The aviation community and the public want to know what happened so we can prevent it from happening again.”