New Orleans police chief Eddie Compass has unexpectedly resigned, four weeks after law and order broke down in the city following Hurricane Katrina.
Compass: 26 years with the police
He gave no reasons for his decision and refused to answer reporters' questions.
Earlier, the police department said it would conduct an investigation into nearly 250 officers who failed to report for duty after the hurricane.
Meanwhile, President George W Bush made a seventh visit to the scene of Hurricane Katrina.
He said he understood the "frustration" of refugees from the stricken Gulf Coast who were still being told they could not "come back to the communities they love".
He urged people to wait until basic services were back up and running.
Meanwhile, the man who led the initial relief effort when Hurricane Katrina hit the US Gulf Coast last month has admitted making some mistakes.
But Michael Brown, who lost his job at the Federal Emergency Management Agency (Fema) in the wake of the hurricane, told a congressional hearing that the mayor of New Orleans and the Louisiana state governor had hindered a co-ordinated response to the disaster.
One congressman said he was stunned that Mr Brown was trying to blame others for the failings of his own agency.
Superintendent Compass said he was resigning after 26 years with the force.
"Every man in a leadership position must know when it's time to hand over the reins," Mr Compass told reporters.
"I will ask you to respect my privacy, respect my decision and just respect my right to be by myself."
Pressure to quit?
Standing alongside him, New Orleans Mayor Ray Nagin said: "It's a sad day in the city of New Orleans when a hero makes a decision like this."
The mayor added that the police chief left the department "in pretty good shape and with a significant amount of leadership".
New Orleans was devastated by the impact of Hurricane Katrina
The BBC's Claire Marshall in New Orleans says that publicly, the police superintendent is choosing to go.
But she adds that the timing of the announcement could lead to suspicions that he was put under pressure to leave.
Police officers who failed to show up for work following the hurricane - about 15% of the force - could face disciplinary action for going absent without permission.
However, Deputy Chief Warren Riley said each case would be looked at individually to determine whether the officer had a legitimate reason.
Hurricane Katrina - one of the worst to hit the US - devastated parts of Louisiana and Mississippi on 29 August, causing massive flooding in New Orleans and killing about 1,000 people.