The USA Patriot Act, enacted seven weeks after the September 11 attacks, granted the federal government sweeping new powers to expand surveillance, curtail financing, and deport aliens in connection with terrorist activity. The first major piece of legislation to respond to apparent weaknesses in U.S. national security, the statute expanded the range of aliens who could be excluded or deported from the United States on terrorism-related grounds, while reducing the procedural protections available to them. Under the new law, immigrants “certified” as threats to national security must be held in government custody without bond pending deportation proceedings and removal from the country. Detention could become indefinite for those aliens found to be deportable but whom other countries decline to accept.

As the USA Patriot Act went into effect, several hundred immigrants remained in government detention under a separate emergency order allowing them to be held without charge for an extended period. The lengthy detention of so many aliens, few of whom were suspected of involvement in the terrorist attacks, generated concern that efforts to protect national security in the wake of September 11 had infringed on the constitutional rights of noncitizens. In 2002, civil liberties organizations mounted several legal challenges on behalf of individuals detained after September 11, including a class action lawsuit asking a federal district court to declare the detention of a group of Muslim men unconstitutional.

Numerous legal scholars have addressed the tension between national security and civil liberties posed by new government policies since September 11. In immigration scholarship, law review articles have addressed the mass *detention of noncitizens, the use of racial profiling in immigration enforcement, and expanded secrecy in immigration proceedings. Yet no article published to date has closely analyzed the mandatory detention provisions of the USA Patriot Act or subjected them to detailed constitutional scrutiny.

This Note argues that the USA Patriot Act’s provisions for certification and mandatory detention contravene the Fifth Amendment’s guarantee of due process of law. By denying noncitizens the opportunity for meaningful review of the certification decision, and by authorizing the detention of aliens on substantively inadequate grounds, the USA Patriot Act raises serious constitutional concerns under both the procedural and substantive prongs of the Due Process Clause. Part I of this Note describes the changes to immigration law presented by sections 411 and 412 of the USA Patriot Act, focusing on the establishment of a new “certification” process that triggers mandatory detention for noncitizens. Part II explains that the Due Process Clause of the Fifth Amendment, applicable to citizens and aliens alike, provides a constitutional basis for challenging section 412. Part III presents the procedural due process claim. The Note argues that mandatory detention of certified aliens implicates a protected liberty interest under the Due Process Clause, and that the procedures of section 412 most likely do not pass constitutional muster. Moving to the substantive due process claim, Part IV argues that section 412 wrongfully authorizes the detention of aliens who pose neither a danger to the community nor a risk of flight. This argument draws support in part from recent federal appeals court cases that have invalidated the mandatory detention of aliens convicted of particular criminal offenses.

The argument presented is circumscribed in two ways. First, this Note focuses on the detention of aliens pending the completion of removal proceedings. Therefore, it does not address the additional constitutional problems posed by the potentially indefinite detention of aliens following a final deportation order. Second, the discussion focuses on aliens who have already “entered” the United States and