It isn't "reasonably possible" to say how many Americans have had their emails and phone calls reviewed as part of a four-year-old counter-terrorism law, according to the Office of the Director of National Intelligence.

The law lets U.S. agencies monitor the communications of foreigners outside the U.S. But two senators are questioning whether a loophole allows the storage and search of messages from Americans that are picked up inadvertently while foreigners are being monitored. The intelligence community has repeatedly said it takes steps to minimize the data collected on Americans.

Among the senators' concerns: that the administration hasn't been able to estimate how many people in the U.S. have had their information reviewed under the program.

"We have sought repeatedly to gain an understanding of how many Americans have had their phone calls or emails collected and reviewed under this statute, but we have not been able to obtain even a rough estimate of this number," Democratic Sens. Ron Wyden of Oregon and Mark Udall of Colorado wrote in a minority response to a Senate Intelligence Committee report Friday. The senators said in the report that the Director of National Intelligence had told them it was not feasible to come up with such a number.

The report details the views of the committee regarding a 13-2 vote in May to extend the counter-terrorism law, which is part of the Foreign Intelligence Surveillance Act. The senators were the only lawmakers on the committee to vote against the extension of the law until 2017; it remains unclear when the full Congress might consider the matter.