If a U.S. court grants the FBI's request for Apple to help it unlock a terrorism suspect's iPhone, the case will likely open the door to many similar law enforcement requests, the agency's director said Thursday.

A ruling in favor of the FBI by a California judge "will be instructive for other courts," FBI Director James Comey said during a congressional hearing. A decision in the San Bernardino mass shooting case "will guide how other courts handle similar requests," he added.

Lawmakers questioned the broader impact of the FBI's request, and a judge's initial ruling in favor of the agency, during a hearing on worldwide security threats before the House of Representatives Intelligence Committee.

It's important for law enforcement agencies to have access to criminals' communications, but the FBI request doesn't seem to have a "limiting principle," said Representative Adam Schiff, a California Democrat. "Is there a way through negotiation that we can arrive at cases where it's appropriate to seek this relief and cases where it's not?"

If the FBI wins in this case, it could lead prosecutors to seek tech vendors to help defeat security protections on devices even in misdemeanor cases, Schiff said.

The FBI wants Apple to defeat the iPhone's password security tool that wipes the device after 10 wrong passwords are entered and to kill a short delay limiting how quickly passwords can be input. The agency wants the ability enter an unlimited number of guesses using password cracking software.

Apple has opposed the FBI's request, arguing that law enforcement agencies shouldn't ask tech vendors to help them defeat security controls that protect user data from hackers.

The technology involved in this case, the combination of an older iPhone 5C and its operating system, reportedly iOS 9, may serve as one limit on further law enforcement requests, with tech vendors possibly unable to comply with other smartphones or OSes involved. The case may not be a "trailblazer because of technology being a limiting principle," Comey said.

If the FBI prevails in the case, Apple would be forced to write new software to defeat the password security on the phone, noted Representative Jim Himes, a Connecticut Democrat.

"Where does this authority end?" Himes said. "Is it the position of the FBI that it has the authority to compel the inclusion of code in a new device?"

Himes asked Comey to "paint a very bright line" for where he believes the FBI's authority ends.

"I don't think I can by virtue of expertise, or should, by virtue of my role," Comey answered. "The San Bernardino case is not about us trying to send a message or establish some precedent. It's about trying to be competent in investigating" a terrorism case.

A debate over encryption and law enforcement access to devices needs to happen in the courts, at the Department of Justice, and in Congress, added Comey, who began raising concerns about law enforcement access to encrypted digital communications in late 2014.

"There's a broader policy question that we all have to grapple with that is far broader than any one case," he said.

Some members of the House panel appeared to side with the FBI.

"I would think it's different if you've got two people who've killed 14 folks in a terrorist attack ... versus some divorce lawyer trying to find out who a philandering husband may have been talking to," Representative Lynn Westmoreland, a Georgia Republican. "There is a total difference there, and I think the American people sense that."