President Obama on Tuesday defended U.S. law enforcement's efforts in scrutinizing the Boston Marathon bombing suspects, while federal officials said the FBI had broadened its investigation into possible links between one of the suspects and foreign extremists.

In his first news conference since the Boston attack, Obama said law enforcement agencies had performed in "exemplary fashion." He accused critics of chasing headlines.

His remarks came as the FBI expanded its investigation of the people who had contact with the two brothers suspected of planting two bombs near the finish line of the April 15 marathon. Tamerlan Tsarnaev, 26, was killed in a shootout with police four days after the bombing. His brother, Dzhokhar Tsarnaev, 19, was captured later the same day and faces federal charges that could lead to the death penalty.

Investigators are trying to trace the handgun, a 9mm Ruger, that the elder Tsarnaev used in the shootout. Two law enforcement officials said that an attempt was made to erase the serial number on the gun.

Officials said several "persons of interest" in the United States and Russia are being investigated in connection with the brothers. One focus is the seven months that Tamerlan Tsarnaev spent in Russia in 2012.

During that trip, he may have had contact with two suspected Russian extremists in the country's North Caucasus, according to the law enforcement officials.

In January 2012, the elder brother traveled to parts of Russia where there are Islamic insurgencies. He returned to the Boston area in July.

In the aftermath of the bombing, the FBI and Russian authorities are trying to reconstruct Tsarnaev's activities in Russia. He spent most of his time with family members in Makhachkala, the capital of Dagestan, a Russian province. Family members said he was not involved in any unusual activities.

Officials said the FBI is investigating information that Tsarnaev met with Mahmoud Mansour Nidal, who was suspected of recruiting for Islamist extremists in Dagestan who are fighting the Russians. Nidal died in a gun battle with authorities last May in Makhachkala.

According to the officials, another line of inquiry is possible links between Tsarnaev and William Plotnikov, a Russian Canadian suspected of involvement with militants in the North Caucasus. Plotnikov was killed by Russian police in July and Tsarnaev returned to the United States a few days later, leaving behind a new Russian passport.

The Office of the Director of National Intelligence said Tuesday that its inspector general has launched an inquiry into U.S. counterterrorism agencies' handling of the shards of information they had collected on the brothers in the 18 months leading up to the attacks.