At least four Chinese citizens were detained near a U.S. military post on Guam earlier this month shortly after a successful test of the island’s missile defense system that involved components made on Redstone Arsenal.

According to Guam’s Customs and Quarantine Agency, the three men and one woman were detained in separate incidents after having “illegally entered the vicinity of a military installation.” All four, along with three other Chinese nationals arrested Dec. 10 and 11 on Guam, were found to have entered the island illegally after traveling there aboard a ship inbound from Saipan, the agency said in a news release.

All seven were taken into U.S. custody. Guam is a U.S. territory, as is the Northern Mariana Islands, whose capital is Saipan. The specific installation on Guam where the arrests were made was not identified.