NEW ORLEANS — Four years after taking over the scandal-plagued New Orleans Police Department, Ronal Serpas is stepping down as chief with a mixed legacy: Homicides are at the lowest level in decades, but spates of deadly violence threaten the tourism-dependent city’s image.

Monday’s abrupt retirement announcement raised questions about whether Serpas, 54, was pressured to step down. It came the week after he acknowledged the department had failed to release news about an officer-involved shooting for two days. Furthermore, a new City Council member has said there is increasing dissatisfaction with his job performance.

Serpas said he was not leaving under pressure and ticked off a list of accomplishments including the decrease in homicides, a beefed-up homicide unit and the formation of an anti-gang task force that includes state and federal officials. He also took credit for helping Mayor Mitch Landrieu rebuild a force that was reeling in 2010 from revelations of deadly police violence in the chaos that followed Hurricane Katrina in 2005.

“Between 2005 and 2010, the train had come off the tracks,” Serpas said during a news conference with Landrieu and the new interim police chief Michael Harrison.

Landrieu named Serpas as chief shortly after taking office in May 2010.

The result of a federal probe was a 2012 court-backed “consent decree” in which the city agreed to a host of changes in policies. Rafael Goyeneche, head of the Metropolitan Crime Commission, a private law enforcement watchdog group, credits Serpas with making changes in a corrupt police department culture, including zero tolerance when officers are caught in a lie.

Goyeneche added that investigations improved under Serpas, enabling prosecutors to make better cases.

“He rebuilt the relationship between police and prosecutors into a much more effective relationship,” Goyeneche said.