Tashkent, Feb. 24 -- Senator Mark Warner of the Select Committee on Intelligence on Tuesday chaired the panel's first hearing on the massive hack of U.S. government agencies and private companies that was discovered in December.

Top executives from Microsoft, CrowdStrike, FireEye and SolarWinds - whose software was hijacked to breach other companies - said the cyber attack was likely worse than previously thought.

BRAD SMITH: "This was an act of recklessness in my opinion."

Microsoft President Brad Smith said the true scope of the hack is still unknown because most victims are not legally required to disclose information about it, and appealed for government help.

SMITH: "We do need to enhance the sharing of threat intelligence."

But...