New details about communications between the missing Malaysia Airlines 777-200 and an Inmarsat satellite show an additional, “partial ping” occurred 8 min. after the final hourly contact between the aircraft and spacecraft.

Using an analysis of satellite data furnished by London-based Inmarsat, Britain’s Air Accidents Investigation Branch (AAIB) says evidence of the final, partial signal between the MH370’s L-band terminal and Inmarsat’s gateway Earth station occurred March 8, when the plane vanished en route from Kuala Lumpur to Beijing.

In an AAIB statement posted March 25 by the Malaysia Transport Ministry on its Facebook page, investigators said after primary radar and Aircraft Communications and Reporting System (Acars) transmissions with MH370 ceased, the aircraft’s satellite antenna continued communicating hourly pings to the Inmarsat-3F1 spacecraft, a protocol in which Inmarsat’s gateway Earth station checks in with an idle aircraft terminal to ensure it remains connected to the network.

In the initial days following the aircraft’s disappearance, Inmarsat used six complete satellite pings recorded between 2:11 a.m. and 8:11 a.m. to determine the likely direction of the aircraft, leading investigators to establish arcs north and south of the equator that indicated a range of possible locations for the missing plane. More recently the company analyzed data from other flights that use its satellite network to establish a pattern that narrowed M370’s final path as traveling south over the Indian Ocean, in the vicinity of where search teams have been working for more than a week.

However, AAIB said there is now evidence of a seventh, partial ping, or “handshake” between the aircraft and the ground station that occurred at 8:19 a.m., 8 min. after the last complete ping was transmitted at 8:11 a.m., the AAIB said.

“At this time this transmission is not understood and is subject to further ongoing work,” AAIB said.

A subsequent ping was slated to occur at 9:15 a.m., but “no response was received from the aircraft,” indicating it was no longer logged on to the network, AAIB said.

Sometime between 8:11 a.m. and 9:15 a.m. “the aircraft was no longer able to communicate with the ground station,” according to the statement, which added that the timing was consistent with the maximum endurance of the aircraft.

“This analysis by Inmarsat forms the basis for further study to attempt to determine the final position of the aircraft,” the AAIB said. “Accordingly, the Malaysian investigation has set up an international working group, comprising agencies with expertise in satellite communications and aircraft performance, to take this work forward.”

The ministry released graphics accompanying the statement that depict a large swath of sea in the southern Indian Ocean where the plane is likely to have crashed, assuming it was traveling at a ground speed between 400-450 kt.

So far, sightings of debris in the southern Indian Ocean 2,500 mi. west of Perth, Australia, have not been linked to MH370’s disappearance.