CBP officials told reporters late October that there were at least two large migrant caravans headed to the US - one consisting of approximately 3,500 migrants and another consisting of approximately 3,000 migrants.
The roughly 4,000 migrants plan to proceed to the city of Queretaro and then possibly to Guadalajara, Culiacan, Hermosillo and eventually Tijuana on the USA border.
On Thursday, the group voted to proceed from Mexico City, where they'd been gathered several days, to Tijuana, opting for the longer but less unsafe route.
A US official speaking on the condition of anonymity confirmed to ABC News that border authorities are girding for the first group of about 100 to arrive in Tijuana by the end of the weekend.
The splinter group, which reached Tijuana on Tuesday, did so aboard a fleet of buses.
Democratic Senator Brian Schatz of Hawaii has condemned President Donald Trump's deployment of thousands of US troops to the southern border as a "scandal", accusing the USA leader of forcing troops to miss Thanksgiving with their families for no good reason. Many of them die in the attempt or are kidnapped by organised crime groups.
Undeterred by a harder United States stance against them, the migrants have said they will continue the journey so that they can claim asylum.
Because the route poses a host of dangers, such as attack by criminal gangs, numerous migrants say they feel safer travelling in numbers.
About 5,800 active duty troops are assigned to the border mission.
President Donald Trump, who has taken a firm stance against the caravan he calls an "invasion", has order the deployment of up to 9,000 troops to the border.
"Deploying troops to our own border for no national security reason is one of the biggest scandals of this Presidency", Schatz said in a tweet on Sunday, referring to the deployment being carried out in anticipation of the arrival of several caravans of Central American migrants heading towards the US border. "But we are capturing the costs", he said.
"At the end of the day, and I say this sadly as a citizen of a free republic ... unfortunately we've gotten into a position where everything has to go to the Supreme Court on immigration before a final rational determination can be made on it", the attorney laments.