In a case that is swirling with uncertainties, a few pieces of evidence have stood apart for seeming reliability. Among them was the revelation last Saturday by Malaysia Prime Minister Najib Razak that his country’s investigators, in collaboration with U.S. authorities, had analyzed an electronic ping that MH370 had broadcast to the Inmarsat satellite at 8:11 a.m. on the morning of the disappearance. Based on this data, the investigators had determined that at that moment MH370 must have been somewhere along one of two broad arcs: one which passed through Central Asia, and the other of which covered a swath of largely empty Indian Ocean, far to the south.

The revelation left a burning question unresolved: what about the six earlier pings, which had been exchanged between the aircraft and the satellite about once per hour? Could any position data be deduced from them?

Today, Inmarsat revealed some crucial information. “The ping timings got longer,” Inmarsat spokesman Chris McLaughlin stated via email. That is to say, at each stage of its journey, the aircraft got progressively farther away from the geostationary satellite’s position, located over a spot on the equator south of Pakistan, and never changed its heading in a direction that took it closer—at least for very long.

The last known position of MH370 before it disappeared from Malaysia military radar was over the Andaman Sea, between the Malay Peninsula and the Andaman Islands. The time was about 2:15 a.m., and coincided with the first of the seven Inmarsat pings.

In order for its flight to have taken it ever farther from Inmarsat, MH370 would have had to have traveled within either of two narrow bands. One pointed north, toward India, Bangladesh, and Burma. The other pointed south, across Indonesian airspace and then across the Indian Ocean west of Australia.

The narrower range of possible flight paths reduces the number of plausible scenarios and suggests avenues of investigation. For instance, some have speculated that whoever was at the controls might have flown out over the open ocean, turned left and headed toward the southern arc in order to avoid passing through any areas of military radar coverage. That route is no longer possible. If the plane did travel south, its path should be detectable on stored Indonesian military radar returns.