(Adds background on criminal case, Mayer subpoena)
WASHINGTON, Nov 8 (Reuters) - Former Yahoo Chief Executive Marissa Mayer apologized on Wednesday for a pair of massive data breaches at the internet company and blamed Russian agents at a hearing on the growing number of incidents involving major U.S. companies.
"As CEO, these thefts occurred during my tenure, and I want to sincerely apologize to each and every one of our users, she told the Senate Commerce Committee, testifying alongside the interim and former CEOs of Equifax Inc and a senior Verizon Communications Inc executive.
"Unfortunately, while all our measures helped Yahoo successfully defend against the barrage of attacks by both private and state-sponsored hackers, Russian agents intruded on our systems and stole our users data."
Verizon, the largest U.S. wireless operator, acquired most of Yahoo Inc's assets in June, the same month Mayer stepped down. Verizon disclosed last month that a 2013 Yahoo data breach affected all 3 billion of its accounts, compared with an estimate of more than 1 billion disclosed in December.
In March, federal prosecutors charged two Russian intelligence agents and two hackers with masterminding the 2014 theft of 500 million Yahoo accounts, the first time the U.S. government has criminally charged Russian spies for cyber crimes.
The charges came amid controversy relating to alleged Kremlin-backed hacking of the 2016 U.S. presidential election and possible links between Russian figures and associates of President Donald Trump.
Special Agent Jack Bennett of the FBIs San Francisco Division said in March the 2013 breach was unrelated to the one Yahoo disclosed in December and that an investigation of the larger incident was continuing.
"We now know that Russian intelligence officers and state-sponsored hackers were responsible for highly complex and sophisticated attacks on Yahoos systems," Mayer said on Wednesday.
Russia has denied trying to influence the U.S. election in any way.
The Senate Commerce Committee took the unusual step of subpoenaing Mayer to testify on Oct. 25 after a representative for Mayer declined multiple requests for her voluntarily testimony. A representative for Mayer said on Tuesday she was appearing voluntarily.
Mayer became CEO of Yahoo in 2012. One of the earliest employees of Google, now known as Alphabet Inc, she was the internet search company's first female engineer and led various businesses at Google including the design of its flagship search engine.
(Reporting by David Shepardson; Editing by Susan Thomas)