Widow Pei Xia Chen holds a photo of slain New York Police Department officer Wenjian Liu as his casket departs his funeral in the Brooklyn borough of New York January 4, 2015. [Photo/Agencies]

NEW YORK - Tens of thousands of law enforcement officers from across the country gathered on Sunday for the funeral of the second of two New York City policemen killed last month in an ambush that galvanized critics of Mayor Bill de Blasio.

Family members, politicians, police brass and other mourners filed solemnly into a Brooklyn funeral home on a gray morning to honor Wenjian Liu, believed to be the New York Police Department's first Chinese-American officer killed in the line of duty.

A sea of blue uniformed police officers stood in silence outside. Police Commissioner Bill Bratton, who shook hands with many of the rank and file before entering the building, had urged them to do nothing during the services to steal from the "valor, honor and attention" that rightfully belonged to the slain officer.

At last week's funeral for Liu's partner, Rafael Ramos, some of the uniformed police officers assembled outside the church showed their disdain for Mayor de Blasio by turning their backs when he began his eulogy.

"A hero's funeral is about grieving, not grievance," Bratton wrote in a memo to officers.

The murder of Liu, 32, and Ramos, 40, triggered a backlash in support of law enforcement, following nationwide demonstrations last year over the use of force by police against blacks and other minorities.

The deaths of unarmed black men in Ferguson, Missouri, and New York City, in encounters with white officers rekindled a national debate over racial relations and law enforcement.

The tenor of that debate shifted when Liu and Ramos were shot as they sat in their squad car in Brooklyn by a killer who said he wanted to avenge the deaths of the two unarmed black men.

In New York, the murders frayed already strained relations between the police force and de Blasio. The head of the largest police union said the mayor contributed to the political climate that led to the killing of Liu and Ramos.