At a time when heightened fears of government surveillance coincide with growing anxiety about global terrorism, the next top U.S. law-enforcement officer will face daunting challenges to balance protection of Americans' security and the right to privacy.
President Barack Obama says he has not yet chosen a successor to U.S. Attorney General Eric Holder, who announced his resignation on Thursday, but whoever takes on the job for the last two years of the tenure will have no easy task.
War against Islamic State militants in Iraq and Syria has raised fears that American Islamist fighters returning from the Middle East could plot attacks on U.S. soil, U.S. officials say.
At the same time, the Justice Department and other agencies must keep a watchful eye on the threat of home-grown U.S. extremists, an increasing source of concern to law-enforcement officials across the country.
But even with the focus on stepped-up counter terrorism, the next attorney general will also need a deft political touch to deal with Americans' concerns over wide-ranging government surveillance exposed by former spy agency contractor Edward Snowden.
"The challenges that the next attorney general will have to balance are enormous," said Julian Zelizer, a national security expert at Princeton University. "With the new threats emerging overseas and all of the unfinished business (at home), it's hard to believe the next two years will be any easier than the last six."
The White House is moving carefully to identify a replacement for Holder, who announced his resignation on Thursday. U.S. solicitor-general Donald B. Verrilli, former White House counsel Kathryn Ruemmler and Manhattan U.S. Attorney Preet Bharara are seen as possible contenders.
Verrilli's advantage may be his expertise in telecommunications law, which is important when surveillance has become such a contentious issue. Ruemmler worked closely with Obama for three years as his top legal adviser.
Obama's Republican critics, who often had tense dealings with Holder, are already threatening a protracted confirmation fight that could be further complicated by the November congressional mid-term elections.
FIGHTING ISLAMIC STATE
Among the next attorney general's highest-profile responsibilities will be working with other U.S. agencies to ensure that dozens of Americans who went to Syria to fight with rebel groups do not return home to mount attacks.
U.S. officials also see the risk from hundreds of other militants with European passports who might target U.S. and European interests.
To combat this threat, the new attorney general is expected to pursue a campaign launched by Holder to lobby other countries for legal changes that would make it easier to jail suspects on terrorism-related charges, a U.S. law-enforcement source said.
In a speech in Norway in July, Holder suggested other countries should learn from U.S. undercover "sting" operations against suspected militants, a method that many other Western nations have been reluctant to use.
The Justice Department will face a growing need for counter terrorism investigators to track so-called "lone wolves," individuals who have become radicalized by surfing extremist websites rather than by direct contact with known militants, a senior U.S. security official said.
That is believed to have been how two ethnic Chechen brothers accused in last year's Boston Marathon bombing were drawn to violence. The official said that since then, U.S. law enforcement had made only modest progress in this area.
At the same time, the next attorney general is sure to come under close scrutiny over how the Justice Department deals with the tricky task of collecting critical intelligence on terrorism suspects while respecting ordinary Americans' privacy rights.
Snowden's disclosures of the National Security Agency's mass collection of Americans' phone and email records, as well as U.S. spying on allied foreign leaders, have forced a series of reforms. But U.S. intelligence officials say they are still recovering from the damage to the spying operations caused by the revelations, which caused an uproar at home and abroad.
Whoever replaces Holder will also share in another policy headache - the internationally condemned prison for foreign terrorism suspects at the U.S. Naval Base at Guantanamo Bay in Cuba. Obama inherited the prison from his predecessor George W. Bush and has repeatedly vowed - and failed - to close it.
An effort by Holder early in Obama's tenure to bring the inmates to trial on the U.S. mainland foundered amid a firestorm of political opposition, and there appears to be little chance for reviving such an effort any time soon.
It remains to be seen whether Holder's successor will continue an aggressive Justice Department pursuit of the sources of information leaks to the media, under which prosecutors have seized some journalists' phone and email records.
