Tear gas is a chemical weapon banned in war. On Sunday, U.S. Customs and Border Protection agents deployed the chemical agent at migrants over an international border.
Border patrol officials said they used the tear gas at the San Ysidro port of entry after migrants threw rocks at them.
Authorities also appeared to use a flash bang, which makes a loud noise but does not fire a projectile, at a group of people who were trying to get through a section of the border fence, according to the L.A. Times.
Now, officials in both Mexico and California want answers from the Trump administration and are assessing possible legal actions that could be taken in the matter.
WATCH: Trump explains why tear gas was used at Mexico border
Tear gas considered a method of warfare depending on where it’s used
Tear gas is a chemical weapon and is illegal for international use, including on the battlefield. The 1993 Chemical Weapons Convention, which the U.S. has signed, says that although tear gas has no long-term effects when used properly, the substance is banned in battle since it’s difficult to distinguish from more dangerous agents in the fog of war.
However, tear gas is not illegal to use domestically.
CN, or 2-chlorobenzalmalononitrile, is one of the most common types of chemical agents in tear gas. CN gas was used the First World War and in tunnels during the Vietnam War.
On U.S. soil, tear gas is allowed to be used in domestic law enforcement to control riots. But it’s considered a chemical weapon if used as a method of warfare.
However, U.S. soldiers threw tear gas canisters into Mexico, meaning they were no longer on domestic ground. Is this legal?
WATCH: Migrants from caravan register in Mexico to work instead of risking rejection at U.S. border
“Is this legal? Is this moral? Is this accurate? What is happening here?” tweeted Sen. Brian Schatz, D-Hawaii, on Nov. 25.
According to the Organization for the Prohibition of Chemical Weapons, the implementing body for the Chemical Weapons Convention, member states are allowed to possess riot control agents and use them for domestic law enforcement purposes.
“Riot control agents are intended to temporarily incapacitate a person by causing irritation to the eyes, mouth, throat, lungs and skin,” the group said on its website.
WATCH: Overcrowding and bad hygiene conditions prompting health concerns at Mexico’s migrant caravan
U.S. Customs and Border Protection said on Twitter that tear gas was used “because of risk to the agents’ safety.”
On Monday, Trump defended the use of the tear gas, saying it was necessary as a defence tactic.
U.S. Customs and Border Protection also said Monday that the use of tear gas will be reviewed.
WATCH: Tear-gassed mother recounts U.S. border chaos
Can it cause damage?
“Tear gas, under the Geneva Convention, is characterized as a chemical warfare agent and so it is precluded for use in warfare, but it is used very frequently against civilians,” Sven-Eric Jordt, a nerve gas expert at Yale University School of Medicine, told National Geographic. “That’s very illogical.”
Jordt said there are many examples of people who have suffered severe injury and burns, especially in enclosed environments or city streets with multi-storey buildings. He said tear gas is also problematic for people with asthma or other conditions, who can have very severe reactions to the substance.
“Tear gases are very serious chemical threats. I think it is very problematic to use them,” he said. “Law enforcement has to weigh the risk of tear gas injury of bystanders against gaining control in a riot situation under the assumption that rioters break the law.”
The Coalition for Humane Immigrant Rights, CHIRLA, an immigrant rights organization in California, condemned the use of tear gas against immigrants at the border.
“It is a despicable act on the part of the Trump administration and CBP officials to attack defenseless women and children firing tear gas, a chemical agent, at them,” said Angelica Salas, executive director for the organization, in a statement.
“These are human beings who are reaching a point of desperation because their asylum claims are being processed at a snail’s pace or not at all.”
British aid group Oxfam said in a statement the use of tear gas was shameful.
WATCH: About 100 migrants have been deported following rush to U.S. border
“Images of barefoot children choking on tear gas thrown by US Customs and Border Patrol should shock us to our core,” Vicki Gass, Oxfam America Senior Policy Advisor for Central America said.
Democrats and other critics called the use of tear gas an overreaction, and questioned the idea of keeping the migrants in Mexico to make asylum claims there.
Mexico’s foreign ministry called on Monday for “a full investigation” into what it described as non-lethal weapons directed toward Mexican territory the day before, a statement said.
WATCH: Trump says migrant caravan uses children as protection when rushing border
California Attorney General Xavier Becerra is assessing whether the state can take legal action over the Trump administration’s use of force against the migrants and future threats to shut the border with Mexico, he said in an interview with Reuters on Wednesday.
“We have been approached by folks who have expressed complaints,” Becerra, who is the son of Mexican immigrants, said. “We are monitoring what’s occurring.”
— With files from Reuters