Magistrate Judge Deborah Robinson decided there was too much of a flight risk for Stewart Nozette, 52, of Chevy Chase, Md., to be free while he awaits trial.
Nozette pleaded not guilty to charges of attempted espionage, following his arrest last week. He is accused of seeking $2 million for selling secrets to an undercover FBI agent posing as an Israeli intelligence officer. The Justice Department could seek the death penalty.
Nozette had high-level security clearances during decades of government work on science and space projects. He was known primarily as a defense technologist who had worked on the Reagan-era Star Wars missile shield effort formally named the Strategic Defense Initiative.
Because he knows so many secrets, including about the nation's nuclear missile program, Attorney General Eric Holder has ordered special communications restrictions placed on him while he's in jail, authorities said.
"The defendant is himself a walking safe deposit box of classified information," said prosecutor Anthony Asuncion. "He is now a treasure trove of some of our most sensitive matters."
During the hearing, Asuncion played video excerpts of an undercover sting operation against Nozette in which the scientist lounges on a hotel room couch, discussing the possibility of having to flee the country if he comes under scrutiny from U.S. officials.
Nozette looked starkly different as he sat in court Thursday. Wearing baggy, black-and-white striped jail clothes, he stared passively as the screen showed him eating and laughing with the undercover agent.
Asuncion said Nozette told the agent he had passed classified information to Israel in the past. Nozette is not charged with doing so.
"He told the agent that he had indeed communicated classified information," Asuncion said. "He had admitted to the agent actual espionage."
Nozette's lawyer, John Kiyonaga, said there was no basis for that accusation, and noted the government's charges don't contain any such allegations. He also argued the video recordings were misleading because they left out significant parts of a longer conversation.
The dispute centered on one portion of Nozette's secretly recorded meetings with the undercover agent.
According to prosecutors, Nozette was paid more than $225,000 by a company that was wholly owned by the Israeli government and spoke to them regularly. In court Thursday, Kiyonaga identified the company as Israel Aircraft Industries.
During one of his secretly recorded conversations with the undercover FBI agent posing as an Israeli intelligence officer, Nozette said: "I thought I was working for you already. I mean, that's what I always thought, (the foreign company) was just a front."
Prosecutors also say Nozette kept a stash of gold Krugerrand coins worth tens of thousands of dollars in a safe deposit box in California - more evidence, they say, of his risk of flight.