What the US is watching that could signal Russia is launching an invasion of Ukraine

It might be a cyberattack targeting Ukraine's energy grid. Or the movement of Russian battalions and tanks along the border and into firing range. Or perhaps even a long-range missile attack.

They're all potential signs US and European officials are closely monitoring that could reveal President Vladimir Putin is moving forward with an invasion of Ukraine. With more than 100,000 Russian troops amassed along the Ukrainian border -- a number that continues to grow -- military and intelligence officials are scrutinizing the latest tactical maneuvers to try to anticipate when Moscow might shift from threatening an invasion to launching one.

US officials say they're closely monitoring cyber intrusions, like the attack that hit Ukraine's government last month. They're watching not just Russian troops that amass on the border but where they're positioned, and they're keeping an eye on what Russia is doing with its equipment like tanks that would be central to any ground invasion.