(Reuters) - Gunmen who killed four men and wounded six others at a backyard football-viewing party of mostly ethnic Hmong had targeted the residence in Fresno, California, possibly in retaliation for a previous altercation, police said on Monday.

The Sunday night shooting rampage shook the close-knit neighborhood, home to mostly Latino and Southeast Asian families, and prompted police to form a task force to look at the activities of Asian gangs in the central valley hub about 160 miles (260 km) southeast of San Francisco.

“Two of my officers were actually covered in blood and had to be decontaminated, go shower and change their uniforms based on what they landed in and were trying to fix,” Fresno Police Chief Andy Hall told reporters.

“They are hurting a bit in that neighborhood right now and officers are hurting too,” Hall said. “We’re going to do everything we can to get this matter resolved on all levels.”

None of the people at the party had any known affiliations with gangs, Hall said, describing the gathering as a “very peaceful, very quiet” family event of about 30 people before the two gunmen showed up.

The gunmen made their way into the yard through an unlocked gate and began firing with semi-automatic pistols shortly before 8 p.m. local time.

“What I can tell you is, this was not a random act. It appears this incident was a targeted act of violence against this residence,” Hall said.

He said most of the guests were Hmong. The slain victims were all men aged between 25 and 35.

The Fresno Bee newspaper reported that Hmong make up nearly five percent of the city’s population and that two of the victims, including one of the deceased, were well-known singers in that community. Several children were at the home but were indoors when the shooting began and were not hurt.

None of the people at the home were armed and all had been cooperative with detectives, Hall said. He said they were not able to identify the attackers because it was dark and only described flashes of light as the weapons fired.

Some of the party-goers may have been involved in a disturbance in the Fresno area the previous week, Hall said, and investigators were trying to determine if the two incidents were related.

“We’ve got a single focus within the organization and this is not going to be tolerated in the Southeast Asian community or in any community,” he said.

The 2010 census showed there were 91,224 Hmong living in California. The United States recruited the Hmong to fight communism in the late 1960s, when the Vietnam War spread into Laos. Many of them fled to the United States after a Communist government came to power in Laos in the early 1970s.