President Trump Monday threatened to send Americans to a notorious prison in El Salvador, asking President Nayib Bukele to “build about five more places.”

Sending American prisoners to foreign jails would violate the Constitution’s Eighth Amendment, which bars “cruel and unusual” punishments, according to criminal justice advocates.

It’s not the first time Trump has threatened to send U.S. citizens to Salvadoran prisons. His comments Monday came while meeting with Bukele, who said returning a wrongly deported Maryland man would be “preposterous” despite a Supreme Court order to facilitate his return.

Human rights observers have said the Salvadoran prison is overcrowded, and have documented instances of inhumane conditions and torture.