WASHINGTON — Homeland Security Secretary Janet Napolitano said this morning that her agency knew of alleged Boston bomber Tamerlan Tsarnaev’s trip to Russia last year even though his name was misspelled on a travel document.

A key lawmaker had said that the misspelling caused the FBI to miss the trip.

Napolitano’s disclosure came as news to Sen. Lindsey Graham, R-S.C., who told the secretary that it contradicted what he’d been told by the FBI.

“They told me that they had no knowledge of him leaving or coming back so I would like to talk to you more about this case,” Graham told Napolitano as she testified before the Senate Judiciary Committee on sweeping immigration legislation.

Napolitano said that even though Tsarnaev’s name was misspelled, redundancies in the system allowed his departure to be captured by U.S. authorities in January 2012. But she said that by the time he came back six months later, an FBI alert on him had expired and so his re-entry was not noted.

“The system pinged when he was leaving the United States. By the time he returned all investigations had been closed,” Napolitano said.

The Russia trip is now seen as potentially important to determining how and when Tamerlan Tsarnaev, the older Boston bomber who died in a firefight with police, apparently became radicalized, and whether he had ties to others.

Tamerlan Tsarnaev was investigated by the FBI at Russia’s request and his name was included in a federal government travel-screening database after that. By the time of the flight Tsarnaev would have faced no additional scrutiny because the FBI had by that time found no information connecting him to terrorism.