Flying helicopters low and slow over the Tucson desert in Arizona, the FBI has been using "signal sniffers" to try to locate Nancy Guthrie's pacemaker.
As the search for the 84-year-old mother of US Today show anchor Savannah Guthrie entered its third week, investigators took to the sky with advanced bluetooth technology.
They were hoping to pick up signals emitted from the device implanted in Ms Guthrie's chest to help trace her whereabouts, US media outlets NewsNation and Fox News reported.
A digital forensic expert says pacemakers can provide clues in a missing person's case, but there are limitations.
'Pacemaker a sensor on the body'
Pacemakers are small, battery-powered devices inserted into the chest and send continual electrical signals to the heart to keep it in a regular rhythm.
These days, many pacemakers connect via bluetooth to a person's mobile phone to transmit data about the heart's performance.
Pacemakers do not have any kind of built-in GPS, but use Bluetooth Low Energy (BLE), which emits electromagnetic signals that could be detected.
Matthew Sorell, a digital forensic expert from Adelaide University, said it had become common practice to attempt to track victims or missing persons through their devices.
"Whether it's a mobile phone or a smart watch or a plug-able device," Dr Sorell said.
"There's now a range of health technologies, including pacemakers, glucose monitors, heart rate and blood pressure monitors, where effectively you're able to put a sensor on the body, and it connects to your phone."
Slow, low sweep of the area
Nancy Guthrie was last seen at her Tucson home on January 31 and was reported missing the following day.
Authorities say her blood was found on the front porch.
Purported ransom notes were sent to news outlets, but two deadlines for paying have passed.
Over the weekend, local media reported a Pima County Sheriff's Department helicopter flying low in a grid search around the area of Nancy Guthrie's home.
The helicopter had reportedly been fitted with a small device known as a "signal sniffer", described as "sophisticated FBI technology" to help pick up her pacemaker's signal.
Maureen O'Connell, a former FBI special agent, said the agency was likely employing a variety of technologies to find Nancy Guthrie.
"With regards to this particular tool … I do know that the pacemaker sends off a very short-distance alert … so they're going to have to get really close," Ms O'Connell told NewsNation.
Dr Sorell said the range of a pacemaker signal was generally about 10 to 30 metres, so investigators would need to use a high-gain antenna tracking slowly back and forth from the air.
High-gain antennas increase signal strength by focusing radio energy into a narrow beam.
"You basically need to sweep slowly. If the device can see that radio signal, you need to give it about 10 to 15 seconds to respond," Dr Sorell told the ABC.
"So they're essentially sending out a paging signal, looking for that device. If it's in range, then it will hopefully respond in time."
Any data transmitted from the pacemaker would only provide insights into Ms Guthrie's location, not the state of her health, Dr Sorell said.
Pacemaker disconnected from phone
Authorities believe Nancy Guthrie went missing during a 41-minute window in the early morning of February 1.
It was about 1:47am when her doorbell camera was disabled.
Last week, the FBI released surveillance videos of a person wearing a handgun holster outside Nancy Guthrie's front door the night she vanished.
Her pacemaker was disconnected from her phone about 2:28am, authorities said.
Although Nancy Guthrie's phone was still in her house, Dr Sorell said there would be no way to access the pacemaker's data while it was untethered.
"The pacemaker is specifically paired to a specific phone," he said.
"It doesn't allow for that sort of remote connection. So having access to the phone at this point isn't helpful."
Savannah Guthrie, co-anchor of the popular NBC News morning show Today, has posted several video messages with her brother Camron Guthrie and sister Annie Guthrie pleading for their mother's return and appealing for the public's help in solving the case.
In one video, they expressed a willingness to meet ransom demands.
Law enforcement and family members have described Nancy Guthrie as being in frail health and in need of daily medication to survive.
Pima County Sheriff Chris Nanos told Reuters on Friday, local time, no proof of life had surfaced since the suspected abduction, but he was quick to add: "There's not been any proof of death either."
ABC/Wires
