The FBI conducts a “substantial” number of warrantless queries for Americans’ e-mails and phone calls in a special database of intercepted communications, but it does not track exactly how often, an intelligence official said in a letter released Monday.

The letter to Sen. Ron Wyden (D-Ore.) also reported that the NSA last year used 198 e-mail addresses or phone numbers of Americans to search the data for communications that might yield foreign intelligence — a low enough number, Wyden said, that the agency should be able to get a warrant for each one.

The letter, written by Deirdre M. Walsh, legislative affairs director for the Office of the Director of National Intelligence, responded to demands Wyden made at an oversight hearing in June for details on intelligence agencies’ searches of data collected through U.S. companies without individualized warrants.

The data are collected under Section 702 of the FISA Amendments Act, a 2008 law that targets foreigners located overseas.

Walsh was addressing concerns raised by Wyden and other lawmakers that U.S. intelligence agencies are combing the Section 702 data for Americans’ communications without a warrant, in what they call “backdoor” searches.

U.S. officials have defended the practice, saying they should not be required to obtain a warrant to search data they have already gathered lawfully. The queries, Walsh said, “allow an agency to rapidly and efficiently locate” foreign intelligence information.

But the FBI’s inability to provide a number, Wyden said in a statement, “shows how flawed this system is and the consequences of inadequate oversight.”

The findings, he said, “raise questions about whether the FBI is exercising any internal controls over the use of backdoor searches including who and how many government employees can access the personal data of individual Americans.”

Walsh said the FBI may query the data to find information that could help counter terrorist plots and other national security threats inside the United States. The bureau may also search the data for evidence of a crime, she said. Its searches are carried out under court-approved rules to protect Americans’ privacy, she said.

Walsh said some of the 198 “identifiers” approved by the NSA for querying may have been used multiple times. “NSA does not track the actual number of content queries made,” she said.

She reported that the agency also carried out 9,500 queries for metadata, such as communication date and time, using U.S. person identifiers.

The CIA conducted fewer than 1,900 searches of Section 702 data using Americans’ e-mail addresses and phone numbers, she said.

Wyden said in his statement that the Foreign Intelligence Surveillance Court, which oversees the law, reported that the NSA acquires more than 250 million Internet communications every year using Section 702.

“So even if U.S. communications make up a small fraction of that total, the number of U.S. communications being collected is potentially quite large,” he said. “The scale of this activity seems to be something that could be conducted pursuant to probable cause warrants.”

In June, the House overwhelmingly voted to require a warrant for such searches. Wyden said an exception could be made for searches for communications of people who are believed to be in danger.