Mobile phone tracking is a controversial subject. After the riot at the U.S. Capitol on January 6, 2021, authorities were able to use cell phone records to determine who was present.
Whether we are actively using them or not, our cell phones are constantly pinging nearby cell towers, emitting a signal then receiving one back. Based on the strength of the emitted signal and the time it takes to reach the cell tower, the location of the phone can be determined.
You may have noticed that the antennas on a cell tower are always arranged in the shape of a triangle. Each antenna array covers a 120° pie-shaped slice of the surrounding area, as shown in the diagram below. By convention, the three sectors are designated alpha, beta, and gamma – α, β, γ.
The diagram below shows how by measuring a cell phone's signal strength and the time it takes for a round-trip signal to be sent and received, the approximate location of the cell phone can be determined.
In the diagram, each concentric circle represents two miles (3.2 km), so a single cell tower can determine that a phone is between eight and 10 miles (16 km) away and within its alpha sector.
When a cell phone is in contact with a second cell tower, the ability to determine its location is improved, as shown in the diagram below. We see that the signal from the cell phone is coming from within the Blue Tower's alpha sector and from a distance of six to eight miles (10-12.8 km) away.
Data from the second, or Orange Tower, shows the signal is coming from its gamma sector at a distance of eight to twelve miles (12.8-19.3 km).
If the cell phone is in contact with a third tower, then its location can be pinpointed to an area of between two to six meters in size. The diagram below shows the signal as coming from the Green Tower's alpha sector and at a distance of between 10 and 12 miles (16-19.3 km).
The FBI Cellular Analysis Survey Team
The FBI maintains a specialized unit called the Cellular Analysis Survey Team, or CAST, that provides analyses of cell phone records and presents the information to law enforcement. We're going to take a look at a recent case in which CAST has played a pivotal role.
By November 2019, relatives of two children — Tylee Ryan and JJ Valow — were getting worried. Neither child had been seen or heard from since September.
When the relatives contacted local law enforcement in Rexburg, Idaho, officers were dispatched on November 26th to the townhouse rented by the children's mother, Lori Vallow, and her new husband, Chad Daybell.
The Daybells assured officers that the children were safe and sound. Seven-year-old JJ was with family friends in Arizona, and 16-year-old Tylee was at Brigham Young University's campus right there in Rexburg.
Returning to the police station, the officers set about confirming whether what they had been told was true. The family friend in Arizona hadn't seen JJ, and BYU had no record of Tylee.
The next day, November 27, 2019, the officers returned to the townhouse to serve a search warrant, but the Daybells were nowhere to be found. Two months went by, then, on January 25, 2020, police on the Hawaiian island of Kauai located Chad and Lori.
Lori was served with a notice to produce Tylee and JJ within five days. When she failed to do so, she was arrested and extradited back to Idaho on March 5, 2020.
A history of bad luck
Bad luck seemed to follow the new couple. Vallow's third husband and Tylee's father, Joseph Ryan, had died suddenly. Lori had married Charles Vallow, her fourth husband, but on July 11, 2019, in Arizona, he had been shot to death by Lori's brother Alex Cox.
Cox had claimed to be acting in self-defense and no charges were brought. When Lori moved to the townhome in Rexburg, which was also where Chad Daybell lived with his wife Tammy, Alex Cox followed, renting a townhouse in the same complex as Lori's. Chad Daybell wrote apocalyptic books about the end of the world.
On September 8, 2019, Tylee, JJ, Lori and Alex made a visit to nearby Yellowstone National Park. Photos show the group apparently enjoying their visit, however, Tylee would never be seen again.
On the night of September 22, 2019, Alex Cox arrived at Lori's home and took JJ back to his home. Early on the 23rd, a neighbor saw Cox carrying an unmoving JJ back to Lori's house.
On October 2, 2019, Lori Vallow purchased two wedding rings on Amazon, then, on October 19, 2019, Chad Daybell's wife Tammy suddenly died in her sleep from what was called natural causes. At the time of her death, Tammy was training for a marathon. Her husband refused an autopsy and received a $430,000 life insurance payout.
On November 5, 2019, Chad married Lori in a ceremony on a beach in Kauai, Hawaii. Photos show Lori wearing one of the wedding rings she had purchased.
On December 12, 2019, in Arizona, Alex Cox died suddenly from a blood clot in his lung. Cox's death made his cell phone available to authorities, and it contained a wealth of information.
CAST goes to work
The FBI's Cellular Analysis Survey Team used the information on Cox's phone to place him four times on a large farm-like property that belonged to Chad Daybell, and where Daybell had previously lived with his wife Tammy.
The data placed Alex Cox on the property on September 6th, 9th, 23rd, and 25th. The Rexburg police had already received statements from Chad Daybell's neighbors stating that they observed fires on his property on September 6th and the 9th.
A probable cause affidavit dated June 10, 2020, and made by Lt. Ron Ball of the Rexburg, Idaho police department included the following information.
"Sunday, Sept. 8, 2019:
6:40 p.m. — Alex Cox leaves west gate of Yellowstone National Park
6:45 p.m. to 7:02 p.m. — Alex Cox is at Buckaroo Bill’s BBQ in West Yellowstone
8:37 p.m. — Alex Cox arrives in Rexburg
9:35 p.m. — Alex Cox leaves his sister Lori Vallow’s apartment
9:43 p.m. to 9:53 p.m. — Alex Cox visits the Maverik gas station on Main Street in Rexburg
10:44 p.m. to 11:15 p.m. — Alex Cox is at Lori Vallow’s apartment
11:44 p.m. — Alex Cox is at his apartment.
"Monday, Sept. 9, 2019:
12:00 a.m. to 12:44 a.m. — Alex Cox is at his apartment
2:42 a.m. to 3:37 a.m. — Alex Cox is at Lori Vallow’s apartment
4:37 a.m. to 8:59 a.m. — Alex Cox is at his apartment
9:21 a.m. — Alex Cox is at Chad Daybell’s property, behind the house near east end of the barn
10:39 a.m. — Alex Cox remains at Chad Daybell’s property
10:47 a.m. — Possible erroneous ping show Alex Cox’s phone in St. Anthony
10:57 a.m. to 11:39 a.m. — Alex Cox is at Chad Daybell’s property
11:52 a.m. to 12:02 p.m. — Alex Cox is at the Dell Taco in Rexburg
11:53 a.m. — Chad Daybell texts his wife Tammy Daybell saying he’s just buried a raccoon in their “pet cemetery”
Alex Cox is mostly at his apartment the rest of the day.
On June 9, 2020, the Rexburg Police Department, FBI, and Fremont County Sheriff’s Office served a search warrant at Chad Daybell’s home. Using the cell phone data, authorities zeroed in on a four-foot by two-foot patch of grass (1.2-0.6 m), which they removed.
Beneath the grass were three large rocks, beneath the rocks was wood paneling, and beneath the paneling was a plastic bag crisscrossed with duct tape. Within the bag were the remains of JJ Vallow.
In another part of the Daybell's yard where Alex Cox's phone had been pinpointed, authorities found a melted plastic bucket containing burnt flesh, and beneath that bucket was a partial human skull. Tylee Ryan had been found, she had been dismembered and her body burned.
Chad Daybell was arrested that same day and both he and Lori remain in jail awaiting their joint trial. Lori has been charged with two felony counts of conspiracy to commit destruction, alteration, or concealment of evidence. If convicted, each charge carries a maximum sentence of five years in prison.
Chad Daybell has been charged with two felony counts of destruction, alteration, or concealment of evidence. Their next court appearance is on March 22, 2021.
How we use cell phone tracking information
The FBI maintains strict guidelines on how it can conduct searches for CSLI, or Cell Site Location Information. As the Vallow-Daybell case shows, a shocking amount of information can be gleaned from cell phone tracking data. Going forward, the issue seems to be about how that information will be used.