By Joseph Tanfani

Hackers backed by the Russian government targeted voting systems in 21 states last year in an effort to undermine confidence in the principle of free and fair elections, U.S. security officials testified on Wednesday.

While the Russian interference in the 2016 campaign has been known for months, it was the first time U.S. officials have said how many states' electoral systems were targeted.

Testifying before the Senate Intelligence Committee, the officials said none of the digital intrusions affected the parts of electoral systems that counted votes _ but that they expect Russia to keep trying.

"I hope the American people will keep in mind Russia's overall aim is to restore its power and prestige by eroding democratic values," said Bill Priestap, assistant director of the FBI's counterintelligence division.

Priestap said the hackers successfully copied data from some states "to understand what it consisted of" and to plan future attacks.

He declined to describe the data that was stolen, saying the Russian interference remains the focus of several FBI investigations.

U.S. officials first found hacking aimed at state systems last August, he said. By late September the efforts were detected in 21 states.

Priestap would not name the states or say which systems lost data, saying the FBI wants to preserve confidential relationships with state and local election officials.

The top Democrat on the committee criticized the FBI for withholding that information.

"I do not believe our country is made safer by holding this information back from the American public," said Sen. Mark Warner of Virginia, the ranking Democrat.

One state official also criticized the federal government for not sharing enough information about the hacking.

So far, no secretaries of state have been cleared to receive classified details of the Russian operation, said Connie Lawson, secretary of state in Indiana and president-elect of the National Assn. of Secretaries of State.

Jeannette Manfra, a cybersecurity official at the Department of Homeland Security, said the decentralized nature of U.S. elections means that an attempt to penetrate state systems and change results would be "virtually impossible" to accomplish without being detected.

The House Intelligence Committee also held a hearing Wednesday to examine the Russian interference in the 2016 election. Jeh Johnson, who was U.S. secretary of Homeland Security during the November election, testified.

The Justice Department has appointed a special counsel. Former FBI Director Robert S. Mueller III also is investigating whether any of President Donald Trump's current or former associates colluded with the Russian effort.

During the presidential campaign, Trump said he hoped Russia would be able to uncover missing emails from Hillary Clinton.

Sen. Martin Heinrich, D-N.M., asked Priestap if he thought Trump had become an "unwitting agent" of Russian interests.

Priestap remained silent. "I don't blame you for not answering that question," Heinrich said.

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