Well the rumours are that the White House is connected via tunnels to the Capitol Subway System and then all the way to the Pentagon and further afield.
No one who knows for certain is likely to talk, but anyway it seems implausible to the extreme that any attack on the white house could actually succeed in reaching the POTUS and holding him hostage before he could be evacuated or moved to an impregnable bunker. The only possible method would seem to be for a deep cover enemy agent to spend years working their way up in the Secret Service and then get assigned to duty actually protecting the POTUS. Given the depth of background checks for those positions that seems implausible in the extreme as well.
Not tunnels, but all the ventilation shafts are man-sized with a switch to turn off the giant fans (note the switch was added later after WH screening of “Galaxy Quest”).
I believe a constitutionally mandated plucky loner, who plays by his own rules and a has no time for the chain of command if it gets in the way of getting the job done, is responsible. I don’t think the supreme court has ruled yet on whether a jaded disposition and an estranged family are also required.