Apparently propelled by fears of a Libyan assassination squad, a little noticed bill which zipped through Congress Wednesday gives the Capitol Police Force the greatly expanded mission of protecting members of Congress anywhere in the United States.

Passed without debate in the adjournment rush, it transforms the 1,100-member force from a parochial agency restricted to the Capitol and environs into one whose officers could be dispatched around the country to protect members and officers of the House and Senate and their families when they travel.

Conflicting explanations were given for its sudden appearance. The Senate sponsor, Ted Stevens (R-Alaska), called it merely a "housekeeping effort" that would give the police power to move in a wider sphere around the Capitol where street crime is common, such as the Union Station area. He denied any connection with the report of Libyan assassins.

However, a House Democrat who asked not to be identified said the reason was concern over reports that a Libyan hit team was being dispatched to the United States to assassinate governent leaders.

Howard S. Liebengood, the Senate sergeant-at-arms who is top supervisor of the police force, agreed that the reported Libyan threat had been a "catalyst" in passing the bill this year, but said the measure had been planned for as long as three years as a precautionary measure in case congressmen are threatened with physical harm.

Congressional leaders were briefed two weeks ago on alleged dangers of a Libyan team sent to kill top U.S. leaders, possibly including some senators, but the details have never been divulged.

Since then, Liebengood said, more security measures have been taken by the police force's protective services unit which is ordinarily assigned only to protect dignitaries who visit Capitol offices. He declined to elaborate.

Under the new law, he said, reported threats would be considered first by the Capitol Police Board, which includes him as chairman, the House sergeant-at-arms and the architect of the Capitol. If a threat is considered serious, he said, plainclothes officers would be assigned to protect the member or officer after the matter was discussed with the leaderhsip of both parties.

There is no immediate plan to assign any officer to such a detail and for the moment no increase in the size of the force is contemplated, he added.

The bill was approved Wednesday morning in the Senate and during the afternoon in the House. Although it contains some sweeping language, it never was debated. House and Senate leaders were aware of its contents but not, apparently, many of the members in either chamber.

The bill authorizes protection for members and officers of both houses and for members of their immediate families.