Some people associated with the Central American migrant caravan, which arrived in Tijuana this week, have crossed into the United States illegally in the last 24 hours, federal officials said Saturday.

A pregnant woman and some children as young as 4 were detected entering the U.S. through a canyon that authorities described in a statement as dark, treacherous and “notorious for human and drug smuggling.”

They were said to have climbed over a dilapidated scrap metal border fence on either side of the San Ysidro Port of Entry.