By David Schwartz

PHOENIX, July 11 (Reuters) - An inmate has been indicted for allegedly plotting to kill a hard-line Arizona sheriff to hold him "personally accountable" for deporting his family members to Mexico, sheriff's officials said on Wednesday.

Samuel Matta, 29, was served with the indictment in prison on Wednesday stemming from an alleged scheme to assassinate Maricopa County Sheriff Joe Arpaio with a high-powered rifle, said Sergeant Brandon Jones, a sheriff's spokesman.

Matta, who authorities say is a documented gang member, planned to act on the plot once he was released from prison, Jones said.

Arpaio, who styles himself as "America's toughest sheriff," is one of the leading proponents of the Mexico border state's crackdown on illegal immigrants passed in 2010, but partially blocked by the courts.

In May, the U.S. Justice Department filed suit against Arpaio accusing him of civil rights violations and saying he and his office intentionally engaged in racial profiling and unlawful arrest of Latinos in violation of their constitutional rights.

The tactics used by the sheriff, who has denied any abuses and has vowed to fight the suit, will come under scrutiny beginning July 19 when another racial profiling lawsuit is heard in U.S. District Court in Phoenix.

Sheriff's investigators said they were tipped to the assassination plot by a jail inmate and learned that Matta was angry that his family had been deported to Mexico from their home in El Mirage, Arizona, west of Phoenix.

The investigation into the alleged plot also revealed plans by Matta and two other inmates to kill another prisoner. Sheriff's detectives said the three men were caught on tape plotting to kill the inmate because he was rival gang member.

Late in June, the U.S. Supreme Court upheld a key part of Arizona's crackdown on illegal immigrants, requiring police to check the immigration status of people they stop.

It threw out three other provisions that had been challenged by the federal government. (Editing by Tim Gaynor)