Magistrate Judge David Hennessy made the decision after lawyers for Jack Douglas Teixeira, 21, asked for him to be released to house arrest pending trial. No trial date has been set.
Teixeira, arrested on April 13, is the primary suspect in the disclosure of sensitive US documents related to the Ukraine war and numerous other topics - an embarrassing leak that has caused US government soul-searching about its failure to protect vital national security secrets.
"Who did he put at risk?" said Hennessy of US federal court in Worcester, Massachusetts. "You could make a list as long as a phone book."
Teixeira appeared at the hearing in an orange jumpsuit and a new buzz haircut.
In deciding to keep Teixeira behind bars, Hennessy said it was not "implausible at all that a foreign government would make overtures to this defendant to get information."
The judge also cited what he said was Teixeira's fascination with guns, which has "an unhealthy component," and a lack of integrity. The judge said Teixeira did not keep his word on a number of occasions.
Later in the 45-minute hearing, Hennessy added: "He's not worried about anybody but himself."
Teixeira leaked classified documents to a group of gamers on the messaging app Discord, according to prosecutors. The leak is considered the most serious US national security breach since more than 700,000 documents, videos and diplomatic cables appeared on the WikiLeaks website in 2010.
Teixeira is charged with one count of violating the Espionage Act, related to the unlawful copying and transmitting of sensitive defence material, and a second charge related to the unlawful removal of defence material to an unauthorised location.
The leaked documents held highly classified information on allies and adversaries, with details ranging from Ukraine's air defences to Israel's Mossad spy agency. US President Joe Biden has ordered an investigation into why the alleged leaker had access to the sensitive information.
Teixeira is being held in Plymouth County jail south of Boston.
While a low-level airman, Teixeira had broad access to military secrets at the Massachusetts Air National Guard's 102nd Intelligence Wing, according to US Justice Department lawyers.
In court papers filed this week, the government said Teixeira's superiors had admonished him twice, in September and October, for his handling of classified information
In the September incident, a superior saw him taking notes on classified information and shoving a piece of paper into his pocket, according to an Air Force memo filed in his court case. A month later, he was admonished again about taking "deep dives" into classified information after asking detailed questions at a briefing.
Nevertheless, his superiors offered Teixeira intelligence training even after the admonishments, and Teixeira bragged online in early January that he had broad access to top secret information.
"I have stuff for Israel, Palestine, Syria, Iran and China," Teixeira said on social media, according to prosecutors. A month earlier, he referred to "all of the s*** I've told you guys I'm not supposed to."
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