Speaker Pelosi waited to make a determination for several weeks because she would have preferred that the Senate take another vote on a bipartisan bill to create an independent 9/11-style commission. Senate Republicans blocked the bill in May, and Senate Majority Leader Chuck Schumer has not called another vote on the measure.
Unlike an independent commission that would not have comprised elected officials, Democrats will control the select committee. There will still be Republican members on the committee, but Democrats will have the majority and therefore, they will also have subpoena power. Select committees are created by a resolution to conduct investigations or consider measures, usually on a specific topic.
It’s unclear whether any Republicans will support the creation of a select committee because they may argue that Democrats would use the committee as a purely political exercise. Previous select committees, like the 2012 select committee on Benghazi led by Republicans, have devolved into partisanship.