John McCain, on a visit to the border town of Nogales, Ariz. with colleagues Chuck Schumer, Michael Bennet and Jeff Flake, bore witness, firsthand, to one of the challenges Congress will face as it attempts to craft immigration legislation: securing the border.

Standing at the country’s Southern border, McCain tweeted:

John McCain         @SenJohnMcCain

Just witnessed a woman successfully climb an 18-ft bollard fence a few yards from us in

Shortly thereafter, he posted an update:

John McCain         @SenJohnMcCain

Border Patrol successfully apprehended her, but incident is another reminder that threats to our border security are real

The four senators are part of the bipartisan “Gang of Eight” leading the legislative effort on immigration reform.