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.

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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.

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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.

During his 2013 campaign for office, the mayor had criticized some of the NYPD’s tactics, including a “stop-and-frisk” policy that critics said was used to harass African-Americans and other minority groups.

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The mayor also offered qualified support for the wave of protests triggered by the two black men’s deaths in New York and Ferguson, Missouri. He said he had talked to his bi-racial son, Dante, about being wary in dealing with police.

In a sign of the police force’s broadening ethnic diversity, observances for Liu are expected to meld Chinese and Buddhist customs with the usual traditions of an NYPD funeral, which date to a time when Roman Catholic men of Irish or Italian descent dominated the force.

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Caiyao Chen, 32, was among a crowd of mostly Asian mourners who pressed against barricades on the sidewalk outside the services. Across from them, police officers with white gloves and dress blues stood guard in silence as they awaited for the funeral to begin.

Chen, who moved to the United States from China in 2000, didn’t know Liu, but he said he was particularly saddened because his death as the only son of a Chinese family irrevocably broke their lineage.

“In Chinese tradition the son carries the blood of the family,” he said. “The family is broken now.”

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(Writing By Frank McGurty; Editing by Larry King)