* U.S. forms task force to aid probe of agent's death
* Raids target arms ring selling to Mexican traffickers (Adding gun arrests, paragraphs 8-11)
WASHINGTON, Feb 17 (Reuters) - The United States is "outraged" by an attack on two U.S. immigration and customs agents in Mexico that left one officer dead, Homeland Security Secretary Janet Napolitano said on Thursday.
Napolitano said Mexico was leading the investigation into Tuesday's shooting death of the Customs and Immigration Enforcement agent, which came amid spiraling violence between Mexican drug cartels and security forces.
Another agent was shot and wounded in the leg during the attack on a major highway.
Napolitano told a Senate committee the United States had formed a joint Justice Department and Homeland Security task force to help the investigation.
"I can speak for the entire administration when I say we are not only saddened by the loss of an agent but we are outraged by this act of violence against an officer of the United States," she said.
"And make no mistake -- justice will be brought to those involved," she said. "We owe nothing less to the memory of Agent (Jaime) Zapata and to those who are still on the job in Mexico. We remain relentless in our efforts to keep our borders secure and to assist Mexico in breaking up the cartels that are plaguing that country."
In a separate news conference, U.S. Attorney General Eric Holder said officials would look into whether the policy barring American law enforcement officers from carrying guns in Mexico needed to be changed.
In another development, police arrested nine people and seized 300 firearms, including assault rifles, in raids in Texas, Arizona and Mexico that targeted an arms ring that allegedly sold weapons to Mexican drug traffickers, authorities said on Thursday.
The sweep came after a federal grand jury unsealed indictments against 17 defendants allegedly involved in shipping to Mexico firearms bought by third-party "straw purchasers" in the United States.
The United States is under pressure to curb the illicit southbound trade in high-powered weapons to Mexico, where more than 34,000 people have died in drug violence since President Felipe Calderon took office in 2006 and pledged to break the powerful cartels.
Arizona straddles a lucrative and heavily trafficked smuggling corridor. Organized criminal networks haul drugs and illegal immigrants north, and spirit guns and cash profits south to Mexico. (Reporting by Deborah Charles and James Vicini in Washington, Tim Gaynor in Phoenix and James B. Kelleher in Chicago; editing by John Whitesides)
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