Immigration raids have been so rare in Puerto Rico that its only detention facility, in an office building next to a mall, can hold only about 20 detainees. Yet federal authorities in the U.S. territory have detained more than 500 people since President Trump took office in January.

The escalation has upset many Puerto Ricans, who are American citizens, and has underscored their uneasy relationship with Washington.

Nearly three-quarters of the detainees have hailed from one country, the Dominican Republic, which lies 80 miles west of Puerto Rico by boat. Many Dominicans share the same ethnic background, language and culture as Puerto Ricans, and the detentions of Dominicans have felt to many Puerto Ricans like an affront.

“It’s a historical aberration,” said Néstor Duprey, an associate professor of social sciences at the Inter American University of Puerto Rico.

Generations of Dominicans, as well as some Haitians, have migrated to the Puerto Rico archipelago on rickety boats from Hispaniola island, starting families and filling critical jobs in housekeeping, home health care and construction. Other than interdictions at sea and occasional raids in the capital, San Juan, federal authorities largely avoided mass immigration enforcement on the island before now.

Puerto Rico and the Dominican Republic “have stronger cultural and linguistic links than, I think, most countries in the world,” said Jorge Duany, an expert on Caribbean migration, citing their accents, Catholicism and shared love of baseball.