Let's say, for the sake of argument, that you're right.
Why do you think "they" would withold this information? What possible motive could there be?
Well, it would tend to make the politicians look bad if they were ineffective in protecting the citizenry from terrorist attacks...
Who is "they"? The political establishment; the administration; controlling leadership in Congress; and local elected and administrative authorities.
Why would they withhold information? Because they are commited to the policy goal of immigration--get more minorities and other immigrants into the United States; immigration quotas are not high enough--so they want lots of new people, legal or not; liberals benefit from new arrivals because they represent groups to whom liberals can exchange promises of benefits for votes. At the moment, the Bush Administration is signed on to this set of goals--in part because the emerging world order seeks to break down national boundaries in all countries by this process.
How does concealing the foreign terrorist particiaption further these goals? The clean clear logical response to domestic attacks of this character is to round up all illegals and deport them; followed by recertifing all visas and residence permits with fingerprints and contact data so that we can control all aliens within the borders; to the end of getting control over the borders of our country--either we have a country or we don't. If we get control over the borders, we make the enemy stop domestic attacks.
Having said all that, will it work? Maybe not. An effort will be made to convince the electorate that it is a domestic however there is so much focus on these events that probably the real perp is going to be disclosed (unlike Oklahoma City; TWA 800 and other events with a similiar direction).