In regards to the ability of Congress to enforce it's own subpoenas one might want to look to the Congressional Research Service Report for Congress dated April 15, 2008
http://www.fas.org/sgp/crs/misc/RL34097.pdf . It takes an in-depth look at Congress' powers including criminal and civil contempt - both pursuable through the Dept. of Justice - and inherent contempt. This last is the ability of Congress to send out the Sergeant at Arms to enforce it's own powers. Unlike criminal and civil contempt, which are punitive, inherent contempt is considered coercive in it's role of bringing those obstructing the legislative process into compliance with the actions of the Congress. Regarding the ability of the Sergeant at Arms to make an arrest and bring the offender to trial before the bar of the Congress:
QUOTE
For example, in McGrain v. Daugherty, which involved a Senate investigation into the claimed
failure of the Attorney General to prosecute certain antitrust violations, a subpoena
was issued to the brother of the Attorney General, Mallie Daugherty, the president
of an Ohio bank. When Daugherty refused to comply, the Senate exercised its
inherent contempt power and ordered its Sergeant-at-Arms to take him into custody.
The grant of a writ of habeas corpus was appealed to the Supreme Court. The
Court’s opinion in the case considered the investigatory and contempt powers of
Congress to be implicit in the grant of legislative power.71 The Court distinguished
Kilbourn, which was an investigation into purely personal affairs, from the instant
case, which was a probe of the operation of the Department of Justice. According to
the Court, the subject was plainly “one on which legislation could be had and would
be materially aided by information the investigation was calculated to elicit.”72 The
Court in McGrain was willing to presume that the investigation had been undertaken
to assist the committee in its legislative efforts.73
See also McGrain v. Daugherty 1927
Kilbourne, mentioned in the above clip from pg.15 of the report, is a case which went before the Supreme Court which appears to set limits on the contempt powers of Congress and is pointed to to suggest those limitations. But subsequent rulings appear to have affirmed the ability of Congress to exercise its contempt powers, as in McGrain.
Only just dug it up and started perusing it myself, but the in-depth nature of the powers of Congress included in this report do appear to lay out the case law which suggests, among other things, that the powers of the Sergeant at Arms of either the House or Senate are not restricted by the physical boundaries of the Capitol as might be suggested by their roles in the Capitol Police and the Board of Officers. Rather, it appears their jurisdiction may be as extensive as is the jurisdiction of Congress itself, and wherever a person obstructing the legislative role of Congress may be they may in fact be well within the jurisdictional territory of the Sergeants at Arms.