Thousands streamed into Mexico City on Monday to seek shelter in a sports stadium on the eve of US midterm elections in which Donald Trump has repeatedly warned about the advancing caravan and ordered thousands of troops to the border.
But around 100 members, including children, did not arrive in Mexico City because they were kidnapped along the route and handed to organised crime groups, Arturo Peimbert, ombudsman for the human rights commission in Oaxaca, told HuffPost Mexico.
Mr Peimbert said they were kidnapped in the state of Puebla as they attempted to reach the capital from Veracruz.
Video: Central American Migrants Hitchhike Towards Mexico City
He said the federal government had put “strong pressure” on transport companies along the caravan route not to pick up the migrants, forcing them to walk through an area he said was “the largest grave in the country, where hundreds of people have disappeared”.
The Los Zetas cartel was likely to be behind the disappearances, he added.
Authorities counted more than 2,000 migrants entering the Jesus Martinez stadium in Mexico City by mid-afternoon on Monday, and a steady flow continued into the night.
The facility has a capacity to hold 6,000 in total, officials said, and four big tents set up for sleeping filled up.
Nashieli Ramirez, ombudsman for the local human rights commission, said the city was preparing to accommodate as many as 5,000 people.
The lead caravan is estimated to have about 4,000 members and several smaller groups are trailing hundreds of miles to the south.
President Trump has made the caravan a major issue in the midterms and portrayed it as a threat to the US, even though such caravans have regularly happened over the years and have largely passed by unnoticed.
He ordered thousands of troops to the US-Mexico border when the migrants were still hundreds of miles to the south, threatened to detain asylum seekers in tent cities, and has insinuated without proof that there are criminals and even terrorists in the group.
In dozens of interviews since the initial caravan set out from Honduras more than three weeks ago, migrants have said they are escaping rampant poverty and violence.
Many are families travelling with small children. Some say they left because they were threatened by gang members or had lost relatives to gang violence. Others say they hope to work, secure a good education for their children and send money to support loved ones back home.
Additional reporting by agencies