[Reporter] They come in early morning.
Running from the border just to the south.
We spotted six.
Teenagers, maybe.
Trying to avoid detection.
Darting into Texas fields.
Headed north.
Destination unknown.
Minutes later, up that same road
a large group rounded up by the U.S. Border Patrol.
Families, mostly Central Americans with young kids dirty from the trek.
Teens travelling alone
hoping they won't be sent back
at least not right away.
For the vast majority who clear criminal checks
getting picked up is the plan.
A risk worth taking to try to claim asylum inside the U.S.
The Rio Grande river on the international border
is only about two kilometres from this spot.
This group of 80 likely crossed over sometime in the last 24 hours.
Scenes like this with increasing numbers are happening along this border daily.
Down the road another round-up.
Part of a new surge.
Twice as many apprehensions over this time last year
building pressure on what critics call Biden's border.
We're going to have to deal with the fact that
families are entering the country and there's no way of controlling that.
you know
as much as they want to they can't
because they can't send them back home
they can't send them back to Mexico.
[Reporter] Sister Norma Pimentel runs sanctuary shelters in Texas.
This one in McAllen
crowded with new asylum seekers
picked up recently and released into the U.S.
Several times a day now
dozens are escorted to the shelter after mandatory COVID testing
Would you call it a crisis currently?
[Sister Norma] Well, it's a crisis when we don't do nothing about it, you know
and so if we can work together in this response
it won't be a crisis.
[Reporter] The U.S. and Mexico butt up against each other here
with only the narrow Rio Grande in between.
The wall, begun under President Bush
and continued under Trump,
has wide gaps.
President Trump enforced a remain in Mexico policy in 2019
keeping asylum seekers out.
Thousands of Central American migrants were stranded
at this former camp in Matamoros, Mexico.
Last month they left.
The camp was closed after President Biden reversed Trump's policy.
Allowing these asylum seekers back into the U.S.
But Aura Cruz hasn't been able to leave yet.
Still stranded with her great-granddaughter Dulce.
The girl's mother was killed back in Guatemala.
A grandson died in the camp.
[Speaking Spanish]
[Reporter] I'm worried about the girls, she tells us
because I could suddenly die
so I'm eager to keep fighting for asylum.
Shelters along Mexico's northern border are now straining to care for the migrants
stuck between two worlds.
In Nuevo Laredo, a Catholic charity took in migrants fleeing their violent lives in Central America
but returned back in Trump's era.
[Speaking Spanish]
It wasn't easy, our lives were in danger, says Edgar.
It's sad because my wife is a teacher and I was a public employee
I never imagined we would emigrate this way.
[Speaking Spanish]
Edgar and Delilah fled gangs in El Salvador over a year ago
paying a smuggler to get them across the river
but they were forced back into Mexico.
They're still on a list of asylum cases
waiting.
Do you think you'll have more chance to get into the United States now?
[Reporter translates] Yes
We've seen on the news a lot of families have already been granted access to the U.S.
We hope that's our case as well.
[Reporter] Back on the U.S. side
surveillance has ratcheted up.
Texas calling up its National Guard to augment border patrol.
Criminals and contraband filter in alongside families.
Near the border roads, we find evidence of the cartels who control the perilous river crossings.
Discarded wristbands.
This one labelled arrivals.
Border officials say smugglers are using these bracelets to identify migrants.
How much they paid
and which cartel they went through.
This spring, migration on the southern border
is fast becoming president Biden's consuming problem.
Even as the U.S. warns don't come
every day more do.
Susan Ormiston, CBC News, Abram Texas.