Seven Chinese nationals were arrested recently for illegally entering Guam around the time of a recent U.S. missile interception test on Dec. 10 and 11, according to Guam authorities.

The Guam Customs and Quarantine Agency said at least four of the seven were found “in the vicinity of a military installation” and that the investigation is ongoing.

The Chinese nationals had arrived on a boat from Saipan, Northern Mariana Islands, according to authorities.

Guam is home to several U.S. military installations, including the Andersen Air Force Base, where the U.S. military conducted a live ballistic missile interception test using a new radar on Dec. 10.

A military aircraft dropped a ballistic missile about 930 miles from Guam, and the interceptor tracked and destroyed it mid-flight at 600 kilometers (373 miles) above sea level before it reached the island.

The first-ever missile interception test on Guam occurred as part of efforts to strengthen defenses in and engage threats aimed at the island or the broader Indo-Pacific region.

“This is a tremendous group effort and provides a glimpse of how organizations within the Department of Defense have come together to defend our homeland Guam now and in the future,” stated U.S. Missile Defense Agency Director Lt. Gen. Heath Collins.

“Collectively, we will use this to build upon and validate joint tracking architecture and integrated air and missile defense capabilities for Guam.”

Guam, a Micronesia island, is strategically important for the United States in the Indo-Pacific as the Chinese communist regime aggravates military tensions in the area with U.S. allies while strengthening its own influence over island nations.

Its strategic location also makes it a potential target for U.S. adversaries, according to the Pentagon. Former congressman Mike Gallagher last year sounded the alarm that Guam is “highly vulnerable” to an attack by the Chinese military because of gaps in missile defense.

U.S. officials and lawmakers have also raised concern about Chinese espionage on U.S. military bases.

Recently, a Chinese national was arrested for flying a drone over a military base in California during a Space Force launch.

In October, five Chinese nationals were charged with covering up their visit to Camp Grayling in Michigan, the largest Army National Guard training facility in the United States, during last summer’s annual Northern Strike training event.

In July, a Chinese student pleaded guilty to espionage misdemeanors for for using a drone to take photos of naval shipyards in Virginia.

Lawmakers have urged law enforcement to shore up detection of this type of “non-traditional” intelligence collection, such as the Chinese regime utilizing civilians including tourists and foreign exchange students.

Frank Fang and Reuters contributed to this report.