Can Americans be locked up on mere suspicion?

Yes! writes Constitutional expert Rob Natelson, author of The Original Constitution, writing for the blog at the Tenth Amendment Center. Commenting on the National Defense Appropriations Act, he carefully spells out what is appropriate to military law, and what the Framers intended when writing the provision for habeas corpus in the United States Constitution. He concludes that the much-discussed Sections 1021 and 1022 would indeed permit American citizens to be held on mere suspicion of terrorism.
To a section that the spin doctors are trying to persuade us was written to protect ordinary Americans, Mr. Natelson writes:

(e) . . . Nothing in this section shall be construed to affect existing law or authorities relating to the detention of United States citizens, lawful resident aliens of the United States, or any other persons who are captured or arrested in the United States.Comment: This provision is sometimes touted as protecting citizens because it preserves existing Supreme Court decisions. The problem is that, as yet, there are no Supreme Court decisions that squarely provide the full measure of habeas corpus protection to citizens or legal aliens accused within our borders. This is true because neither the Bush nor the Obama administration has had the audacity to round up U.S. citizens without our borders and hold them indefinitely without trial.

He then goes on to cite four Supreme Court decisions, and explains why none of them get to the heart of the matter. Then there is this, also ballyhooed as protection for the rights of Americans:

§ 1022: (b) (1) . . . The requirement to detain a person in military custody under this section does not extend to citizens of the United States.
(2) . . . The requirement to detain a person in military custody under this section does not extend to a lawful resident alien of the United States on the basis of conduct taking place within the United States, except to the extent permitted by the Constitution of the United States.

Comment: This section says that the administration is not REQUIRED to keep a U.S. citizen or legal resident alien in indefinite mCilitary custody. But it does not prevent the administration from doing so.

As we warned Nov. 29 and Dec. 16, vigorous application of this act against Americans will turn skeptics into secessionists. But you don't have to take my word for it. Rob Natelson said it and backed it up.