WASHINGTON — Amid questions about whether the F.B.I. missed an opportunity to discover that one of the suspects in the Boston Marathon bombings may have become an extremist, law enforcement officials defended their actions on Monday, saying they had no legal basis to monitor him in the months leading up to the attack.

The agency first looked into the suspect, Tamerlan Tsarnaev, in 2011 in response to a request from Russia, which told the F.B.I. that he “was a follower of radical Islam.” But once investigators closed the background check on Mr. Tsarnaev after concluding that he posed no terrorist threat, a senior law enforcement official said, it would have been a violation of federal guidelines to keep investigating him without additional information.

“We had an authorized purpose to look into someone based on the query we received,” the official said. “You can do a limited investigation based on that request.”

Senior F.B.I. and intelligence officials will be forced to explain to the Senate Intelligence Committee in a classified briefing on Tuesday the steps they took — and did not take — before and after Mr. Tsarnaev returned last July from a six-month trip to Chechnya and Dagestan, predominantly Muslim republics in the North Caucasus region of Russia.