Bruce Sewell, Appleâs general counsel, said in written testimony submitted before the hearing that the F.B.I.âs demand for the company to break into an iPhone that belonged to one of the shooters in the San Bernardino, Calif., attacks that left 14 people dead âwould set a dangerous precedent for government intrusion on the privacy and safety of its citizens.â
The F.B.I. director, James B. Comey, emphasized the importance of law enforcementâs ability to get access to data for criminal investigations.
He said the agency had increasingly confronted cases in which significant evidence resided on devices such as mobile phones, tablets and laptops.
âIf we cannot access this evidence, it will have ongoing, significant impacts on our ability to identify, stop and prosecute these offenders,â Mr. Comey said.
The battle between Apple and federal law enforcement has come to symbolize a yearslong struggle between technology companies and the government over privacy and national security. The F.B.I.âs court order for Apple to provide access to the iPhone used by the San Bernardino attacker, Syed Rizwan Farook, has divided the public and lawmakers over the responsibility of private companies like Apple to protect the civil liberties of users while also assisting the government in efforts to combat terrorism.
The case has drawn stronger support for federal legislation that would more clearly define the governmentâs ability to obtain encrypted data.
âCongress is best-suited to resolveâ questions about encryption and national security, said Representative Bob Goodlatte, Republican of Virginia and chairman of the committee.
âThis particular case has some very unique factors involved and as such may not be an ideal case upon which to set precedent,â Mr. Goodlatte said.
Since the case was made public last month, Apple has become an outspoken critic of the Federal Bureau of Investigationâs demands, which some executives have warned will trample civil liberties.
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In the hearing on Tuesday, Appleâs top lawyer said that breaking into the phone used by Mr. Farook would require that the company write new software to bypass security locks. Mr. Sewell said that the F.B.I.âs order amounted to âa backdoor into the iPhoneâ and that if Apple complied, it âwould set a dangerous precedent for government intrusion on the privacy and safety of its citizens.â
Once that backdoor was created, âhackers and cybercriminals could wreak havoc on our privacy and personal safety,â Mr. Sewell said.
Mr. Comey said the request was limited to the iPhone used by the San Bernardino shooter. He reiterated comments made last week in a separate hearing that encryption was the most challenging issue facing the F.B.I., but he said the San Bernardino investigation should be seen in isolation.
âThis case is not about the F.B.I., itâs not about Apple, itâs not about Congress,â Mr. Comey said, adding that the investigation was still active.
He also disputed characterizations made by Apple and some lawmakers that the F.B.I. was asking Apple to create a âbackdoorâ that could be exploited for hackers and other terrorists to further weaken security for millions of iPhone users.
Rather, Mr. Comey said, the F.B.I. is asking Apple to remove a âguard dogâ standing in their way of obtaining data on the iPhone that could still contain information used for its investigation.
Apple doubted that law enforcement, if successful in its order, would stop at the San Bernardino case. Mr. Sewell said in his written testimony that Mr. Comey and other law enforcement officials had indicated their desire to create a legal precedent to make similar demands to break into iPhones in other cases.
Cyrus R. Vance Jr., the district attorney of Manhattan, for instance, has said he planned to use the San Bernardino order to obtain access to data on cases involving 175 other phones, Mr. Sewell noted.
âWe can all agree this is not about access to just one iPhone,â Mr. Sewell said.
Mr. Vance, who also testified at Tuesdayâs hearing, said Appleâs encrypted operating system, introduced in fall 2014, had frustrated the efforts of local law enforcement to get data that could help them in their investigations. With the vast majority of criminal prosecutions handled by state and local law enforcement, he called for federal legislation to empower police.
âIn the absence of a uniform policy, our nation will effectively delegate the crafting of national security and law enforcement to boardrooms in Silicon Valley,â Mr. Vance said in prepared remarks.
Companies, analysts and government leaders closely watching the struggle between Apple and the F.B.I. say the case could create a precedent for technology companies whose software and devices contain huge amounts of personal data on consumers.
The legal grounds of government data requests is still being debated. A federal magistrate judge on Monday denied the Department of Justiceâs request that Apple extract data from an iPhone from a drug case in New York, a test of the governmentâs use of a centuries-old law for several other cases involving demands for iPhone data.
Also on Monday, Representative Michael McCaul, Republican of Texas, and Senator Mark Warner, Democrat of Virginia, introduced a bill to create a 16-member bipartisan committee called the National Commission on Security and Technology Challenges to explore encryption and cybersecurity legislation.
âThe ongoing Apple vs. F.B.I. dispute is only a symptom of a much larger problem,â said Mr. McCaul, who is also chairman of the House Committee on Homeland Security. âBut we are almost certain to see this scenario repeated unless the larger issue is addressed. Law enforcement clearly needs the ability to gain lawful access to information that can stop future attacks.â