BROWNSVILLE, Texas >> Shelters in a Texas city struggled to find space Saturday for migrants who authorities say have abruptly begun crossing by the thousands from Mexico, testing a stretch of the U.S. border that is typically equipped to handle large groups of people fleeing poverty and violence.

The pace of arrivals in Brownsville appeared to catch the city on the southernmost tip of Texas off guard, stretching social services and putting an overnight shelter in an uncommon position of turning people away. Officials say more than 15,000 migrants, mostly from Venezuela, have illegally crossed the river near Brownsville in the past week.

That is a sharp rise from the 1,700 migrants that Border Patrol agents encountered in the first two weeks of April, according to U.S. Customs and Border Protection officials.

“It’s a quite concerning because the logistical challenge that we encounter is massive for us,” said Gloria Chavez, chief of the U.S. Border Patrol Rio Grande Valley Sector.

The reason for the increase was not immediately clear. Chavez said migrants have been frustrated by relying on a glitch-plagued government app that can allow them to seek asylum at a port of entry. Some migrants who crossed last week cited other motivators, including cartel threats that immediately preceded the sudden increment.

The uptick comes as Joe Biden’s administration plans for the end of pandemic-era asylum restrictions. U.S. authorities have said daily illegal crossings from Mexico could climb as high as 13,000 from about 5,200 in March.

Other cities — some far away from the southern U.S. border — also are grappling with suddenly large influxes of migrants.

In Chicago, authorities reported last week a tenfold increase in the arrival of migrants in the city, where as many as 100 migrants have begun arriving daily and begun sheltering in police stations.

Brownsville is across the Rio Grande from Matamoros, Mexico, where a sprawling encampment of makeshift tents has housed about 2,000 people waiting to enter the U.S.

Recently, some tents were set ablaze and destroyed. Some migrants have said cartel-backed gangs were responsible, but a government official suggested that the fires could have been set by a group of migrants frustrated over their long waiting time.

“It was desperation, the cartel,” said Roxana Aguirre, 24, a Venezuelan migrant who sat outside a Brownsville bus station Friday afternoon. “You couldn’t be on the street without looking over your shoulder.”

In downtown Brownsville, families from Venezuela, Cuba, Haiti and China walked aimlessly, carrying their belongings and talking on their cellphones.

Some waited for their buses while others were in limbo, waiting for relatives before making plans to leave but finding no shelter in the meantime.