Updated at 12:45 p.m. ET

Members of Congress likely won't confine themselves to former special counsel Robert Mueller's report when they question him next week in two open hearings, staffers said.

Mueller, who is reluctant to appear, has said he would confine himself to what he's already written — but the House Permanent Select Committee on Intelligence won't.

The lineup is now set for the second series of Democratic presidential primary debates, at the end of the month.

On the first night, progressive favorites Vermont Sen. Bernie Sanders and Massachusetts Sen. Elizabeth Warren will be on stage together for the first time, while upstart South Bend, Ind., Mayor Pete Buttigieg will be trying to continue his improbable rise.

Updated at 6:12 p.m. ET

Homeland Security acting Secretary Kevin McAleenan says the U.S. has apprehended more than 800,000 migrants attempting to enter the country since last October, calling the numbers staggering and unprecedented, and that the influx has "challenged and overwhelmed every aspect of our border and immigration enforcement system."

Still, McAleenan said DHS "made significant strides in its effort to secure the border and help and protect migrants in our custody."