Stocks are sliding

Geopolitical risk is back in a big way and now stocks are tumbling.

Near 8:00 am ET, US futures were lower across the board after the most recent headlines about a Russian fighter jet that was shot down on the Turkey-Syria border.

Dow futures were off about 105 points, S&P 500 futures were off 13, and Nasdaq futures were down 30.

Futures initially sold off to these levels a few hours ago when headlines about the Russian fighter jet first crossed, and stocks have taken another leg lower following comments from Russian president Vladimir Putin in the last few minutes.

Putin said in a press conference, that the act was a, "stab in the back by the associates of terrorism." Putin added, "Today's tragic event will have serious consequences for Russian-Turkish relations."

And so it seems that geopolitical risks - the most amorphous and ambiguous of looming market risks at any one time - are firmly back on the table on Tuesday.

In an email on Tuesday morning, Peter Tchir at Brean Capital, noted that in August, everything was quiet in markets until it wasn't, and added that it's worth remembering for US investors that, "Our holiday schedule is not necessarily the world's holiday schedule."

US markets will be closed Thursday and open for just a half day on Friday.

In other economic news, the second estimate for GDP is due out at the bottom of the hour, with Case-Shiller home prices, and the Conference Board's latest consumer confidence reading also set for release later this morning.