FISA prohibits this without a court order, either obtained prior to or in emergency cases shortly thereafter, for good reason. It's called the minimization requirement.

The FISA Court was clearly concerned about this in 2003 when it issued its first published decision ever. As EPIC reported:

Although FISA surveillances must have an intelligence purpose, courts allow FISA-obtained information to be used in criminal trials. However, FISA's "minimization" requirement mandates that procedures be implemented to minimize the collection, retention, and dissemination of information about United States persons. Minimization procedures are designed to prevent the broad power of "foreign intelligence gathering" from being used for routine criminal investigations. In a number of instances, however, there are overlaps between foreign intelligence gathering and criminal investigations. One common minimization procedure is what is known as an "information-screening wall." These "walls" require an official not involved in the criminal investigation to review the raw materials gathered by FISA surveillance and only pass on information that might be relevant evidence. The purpose is to ensure that criminal investigators do not use FISA authority for criminal investigations.

In March of 2002, the Attorney General proposed a new regime of minimization procedures. The Foreign Intelligence Surveillance Court rejected these procedures in May, in the first published opinion of that court.

The opinion was later reversed by the secret, one-sided FISA review court. That decision is here (pdf). But in reversing the opinion the Court reaffirmed the requirement of minimization contained in the statute.

....for U.S. persons, FISA requires minimization of what is acquired, retained, and disseminated. The FISA court notes, however, that in practice FISA surveillance devices are normally left on continuously, and the minimization occurs in the process of indexing and logging the pertinent communications. The reasonableness of this approach depends on the facts and circumstances of each case.

By not applying for a court order, the Government can circumvent the minizmization requirements of the statute and engage in whosale monitoring, retention and dissemination of information of Americans, under the guise of searching for information on Al Qaeda members overseas.

Another problem is that our Attorney General, despite promises made at his confirmation hearing, still believes that he is Bush's lawyer, not our lawyer.

Just 13 months ago, at his confirmation hearing, Gonzales vowed that he would "no longer represent only the White House," instead representing "the United States of America and its people." Yesterday, however, he relapsed, referring to Bush at one point as "the client."

The ACLU has more on Gonzales' misplaced loyalty:

"Attorney General Gonzales is supposed to represent the interests of the American people, and not the president. Sadly, the 'people's lawyer' today hid behind the veil of national security and failed to answer simple and legitimate questions from Senators on both sides of the aisle. These were basic questions that would not have compromised ongoing national security operations. This lack of transparency undermines the ability of Congress to conduct much-needed oversight on this controversial program.