Sunday's incident marked an escalation of tensions that have been mounting since groups of Central American migrants began arriving in Tijuana a few weeks ago on their journey to attempt to gain entry to the US.
Some 9,000 members of the migrant caravans are camped in Baja California along the U.S. -Mexico border, plotting their attempts to enter the U.S., according to Mexican press accounts.
At that point, before the group had reached the border, U.S. Customs and Border Protest officers who had gathered on the other side of the fence launched canisters of what a Reuters photographer identified as tear gas. Mexico's Milenio TV showed images of migrants climbing over fences and peeling back metal sheeting to enter.
Image A migrant family runs from tear gas released by US border patrol
US Customs and Border Protection closed road and pedestrian bridges on Sunday at the San Ysidro port of entry, one of the largest land border crossings between San Diego and Tijuana, Mexico.
Children screamed and coughed.
"We ran, but when you run the gas asphyxiates you more", Zuniga told the AP while cradling her 3-year-old daughter Valery in her arms.
CBP added that some demonstrators "attempted to illegally enter the US through both the northbound and southbound vehicle lanes at the port of entry itself".
At times, such responses can involve "less lethal capabilities crossing the border", he said, adding that wind may blow gas in various directions.
As the chaos unfolded, shoppers just yards away on the USA side streamed in and out of an outlet mall, which eventually closed. U.S. helicopters flew low overhead, according to an AFP journalist on the scene. The violent clashes took place near the border crossing separating the Mexican city of Tijuana from San Diego. All northbound and southbound traffic was halted for several hours. "So some people got rocks and threw them because many kids were fainting", he said.
'And once our agents were assaulted and the numbers started growing - you know we had two or three agents at a time facing hundreds of people at a time - they deployed tear gas to protect themselves and protect the border'.
"As I have continually stated, DHS will not tolerate this type of lawlessness and will not hesitate to shut down ports of entry for security reasons", she continued.
Numerous migrants have been in the camp for more than two weeks.
The refugees are legally allowed to apply for asylum in the U.S., but this process has been known to take as long as a year.
Mexico's foreign ministry said it presented a diplomatic note to the US government on Monday calling for an investigation into the use of tear gas directed toward Mexican territory.
The migrants, mostly from Honduras, are part of a "caravan" loudly condemned by President Donald Trump.
Trump has long pushed for a southern border wall and likely feels justified amid the current migrant crisis in shutting down the government over the issue.