Two teens disappeared while on their way to the coast.
Ray Pierce and Jake Ziegler left Sherrills Ford around 1 a.m. on Saturday taking Interstate 77 to Myrtle Beach.
On Tuesday, there were two simultaneous searches, one in Conway, S.C., near the beach and the other along the interstate in Fairfield County, where investigators said a cellphone signal from one of them was last detected.
Deputies said for the first time they have several concerns about the case that they are centering around the teenagers' cellphones.
"I'm not going home until my brother is found," said Kimberly Pierce.
Since the weekend, Kimberly and Michelle Pierce have looked nonstop for their brother and Ziegler.
The two left for Myrtle Beach early Saturday morning, texting and calling friends as they traveled south along I-77.
Deputies said that communication stopped in Fairfield County according to the last ping recorded on cellphone tower No. 12.
"We just learned tower 12 is this one behind me, and at 2:44 in the morning, their cellphone hit off that tower. They were 1.7 miles south of that tower," said Chief Deputy Keith Lewis.
For much of the day, deputies were on four-wheelers searching along the interstate.
High above a state law enforcement division helicopter circled looking for Ziegler's green 2006 Pontiac G-6.
Investigators said its color could make it difficult to spot.
"I'm very worried. Time is ticking and when the sun goes down you can't do much, so daylight is very important," said Michelle Pierce.
Other family members are along the Carolina coast looking as well. They are hoping the two friends didn't get into an accident and will turn up soon. Ray Pierce's younger sister had this message for her big brother.
"'Ray, come home right now' -- that's definitely what I'd say to him and that I love him," said Pierce.
Family members said they have printed hundreds of these fliers putting them up in both North and South Carolina hoping someone saw the teenagers early Saturday morning.