The four Americans kidnapped in Mexico on Friday have been identified, as the FBI, federal partners and local authorities continue to investigate.
The American citizens crossed into Matamoros, in the northeastern state of Tamaulipas just south of Brownsville, Texas, in a white minivan with North Carolina plates on Friday, the U.S. Embassy in Mexico City said said Sunday.
"Shortly after crossing into Mexico, unidentified gunmen shot the passengers of the vehicle. The gunmen herded the four U.S. citizens into another vehicle and fled the scene with them," according to the embassy.
The four Americans have been identified as Latavia "Tay" McGee, Shaeed Woodard, Zindell Brown and Eric James Williams. According to her mother, McGee and her cousin Woodard were among the victims in Matamoros along with their friends Brown and Williams.
A North Carolina driver's license belonging to Williams was also found at the scene, according to sources close to the Mexican investigation.
Barbara Burgess, 54, McGee's mother, told ABC News that her daughter traveled from South Carolina to Mexico for a cosmetic medical procedure and that before the trip, she warned her not to go.
"Ma, I'll be okay," Burgess said McGee replied on Wednesday before setting off.
On Friday, the day of the appointment, Burgess said McGee called to say she was 15 minutes away from the doctor's office. Burgess called McGee later that day but never heard back, she said.
"Her phone just started going to voicemail," she said.
An FBI agent visited Burgess on Friday morning to ask her if she recognized the driver, who has been identified as Williams.
She confirmed with the FBI agent that Williams was the person her daughter and nephew had been traveling with to Mexico.
The U.S. citizens were taken after getting caught in the middle of a confrontation between groups, according to Mexican President Andrés Manuel López Obrador, who said during his daily press conference on Monday that they "crossed the border to buy medicines in Mexico."
"The entire" Mexican government "is there working on this," López Obrador said.
White House press secretary Karine Jean-Pierre said Monday that President Joe Biden is "aware and has been informed" of the four missing U.S. citizens.
The State Department, Department of Homeland Security and Mexican authorities are working together on the case, according to the White House.
"We are closely following the assault and kidnapping of four U.S. citizens in Matamoros, Mexico. These sorts of attacks are unacceptable," Jean-Pierre said. "Our thoughts are with the families of these individuals, and we stand ready to provide all appropriate consular assistance."
"We will continue to coordinate with Mexico and push them to bring those responsible to justice," she added.
State Department spokesperson Ned Price could share little more about the situation during an on-camera press briefing, but said Monday that the department is "closely following" the case.
"The FBI is working very closely with other federal partners and Mexican law enforcement agencies to investigate this," he said. "We're standing ready to provide all appropriate consular assistance. We do also remind Americans about the existing travel guidance when it comes to this particular part of Mexico."
In a statement on Monday, U.S. Ambassador to Mexico Ken Salazar said that various U.S. law enforcement officials are working with Mexican authorities at every level of government "to secure the safe return" of the four kidnapped Americans.
"We have no greater priority than the safety of our citizens -- this is the U.S. government's most fundamental role," Salazar said.
The FBI is now asking for the public's help in identifying who might be responsible and is offering a $50,000 reward, the embassy said.
Anyone with information can contact the FBI's San Antonio Division at 210-225-6741.
ABC News' Anne Laurent, Teddy Grant and William Gretsky contributed to this report.