US investigating whether Chinese citizen charged with flying drone over base committed 'more serious offenses'

But court filings indicate that "the government is continuing to investigate whether ZHOU engaged in additional, more serious offenses."

While Zhou has not been slapped with charges of spying on behalf of China, in a sworn affidavit justifying the arrest warrant, the FBI agent heading the probe into Zhou described their own experience, noting, "I am familiar with tradecraft and techniques utilized by foreign adversarial nations and the efforts those nations undertake to collect intelligence on elements of military infrastructure of their interest."

Zhou was arrested Dec. 9 before boarding a flight to China, according to the press release about the arrest.

"This defendant allegedly flew a drone over a military base and took photos of the base's layout, which is against the law," U.S. Attorney Martin Estrada, noted.

The release, citing an affidavit, reported that a drone was detected above the facility on Nov. 30.

"The drone systems detected that the drone flew for nearly one hour, traveled to an altitude of almost one mile above ground level, and originated from Ocean Park, a public area next to the base," the press release noted. "Base security personnel went to the park, spoke to Zhou and another person accompanying him, and learned that Zhou had a drone concealed in his jacket - the same one that flew over the base.

"Agents later searched Zhou's drone pursuant to a federal search warrant and saw several photographs of Vandenberg Space Force Base taken from an aerial viewpoint. A search of Zhou's cellphone showed Zhou conducted a Google search approximately one month earlier for the phrase 'Vandenberg Space Force Base Drone Rules' and messaged with another person about hacking his drone to allow it to fly higher than it could otherwise."

Nighttime sightings of unidentified aircraft in New Jersey and other parts of the country have been making national headlines, but Americans have been left wanting answers.

Fox News' Lee Ross contributed to this report.