Comey indicated that the debate involving both legal and privacy issues over whether the federal government can compel tech companies to unlock personal devices in the interest of national security is far from over in a briefing with reporters at the Federal Bureau of Investigation.

The Justice Department unlocked the iPhone used by one of the San Bernardino shooters in March using a tool created by an unidentified contractor after Apple refused to bypass the device’s encryption features on grounds it could undermine security for all users.

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Comey said the end-to-end encryption launched earlier this year by messaging service WhatsApp—which is owned by Facebook—is “affecting the criminal work (of the FBI) in huge ways,” though he has no plans to sue the company.

FBI experts have examined about 4,000 devices since October and have been unable to unlock approximately 500, Comey said.

Comey said he thinks none of the 500 phones that the FBI has been unable to unlock since October have the same model-type and operating system as the iPhone used by the San Bernardino shooter, but the agency is trying to figure out a way to use the same tool to unlock other phones.

The identity of the contractor is so closely held inside the FBI that not even Comey knows who it is, a U.S. government source told Reuters last week.

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Comey said he had a “good sense” of the identity of the third-party contractor, but he “couldn’t give you people’s names.”

Comey told reporters the number of Americans trying to join Islamic State has dropped to one per month since August, down from an estimated six to 10 per month in the previous year and a half.

“I think the ISIL brand has lost significant power in the United States,” Comey said.

The FBI currently has more than 1,000 investigations open into people who are in danger of being radicalized by foreign terrorists, according to Comey.