President Obama took questions from reporters at the White House on Tuesday, discussing Syria, the Boston bombings, closing Guantanamo, immigration reform and Jason Collins. (The Washington Post)

President Obama on Tuesday defended U.S. law enforcement’s efforts in scrutinizing the Boston Marathon bombing suspects as federal officials said the FBI has broadened its investigation into possible links between one of the suspects and foreign militants.

In his first news conference since the Boston attack, Obama said law enforcement agencies had performed in “exemplary fashion” in the hunt for the bombers and in investigating one of the suspects before the bombings. He accused critics of chasing headlines.

“Based on what I’ve seen so far, the FBI performed its duties,” Obama said. “Department of Homeland Security did what it was supposed to be doing. But this is hard stuff.”

Obama’s remarks came as the FBI expanded its investigation of the people who had contact with the two brothers suspected of carrying out the April 15 bombings. Tamerlan Tsarnaev, 26, was killed in a shootout with police four days after the bombings. His brother, Dzhokhar Tsarnaev, 19, was captured later the same day and faces federal charges that could lead to the death penalty.

Federal 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 and that experts have been unable to restore all eight digits.

Law enforcement officials said several “persons of interest” in the United States and Russia are being investigated in connection with the brothers. One of the primary focuses is the seven months that Tamerlan Tsarnaev spent in Russia in 2012.

During that trip, Tsarnaev may have had contact with two suspected Russian militants in the country’s restive North Caucasus region, according to the law enforcement officials, who spoke on the condition of anonymity to discuss an ongoing investigation.

The FBI and other agencies began scrutinizing the elder Tsarnaev in 2011 after the Russian security service warned the bureau and the CIA of concerns that he was becoming increasingly radicalized and might travel to Russia to carry out an attack. He was questioned by FBI agents in Boston, but the inquiry concluded that he posed no threat. In January 2012, he traveled to parts of Russia where there are active Islamic insurgencies. He returned to the Boston area in July.

In the aftermath of the bombings, 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 where there is an active Islamist insurgency. Family members said he was not involved in any unusual activities.

But U.S. officials said the FBI is investigating information that Tsarnaev met with Mahmoud Mansour Nidal, who was suspected of recruiting for Islamist militants 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.

Authorities now believe Tamerlan Tsarnaev began his preparations for the bombings after his return to the Boston area last year, a period in which he assembled an extensive playlist of jihadist videos online.

Critics have questioned why the initial FBI inquiry on Tsarnaev was not reopened after his trip to Russia. His return to the United States was registered on a low-level watch list, but no action was taken.

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. The probe will focus on whether information was shared among agencies, including the FBI, the CIA and the National Counterterrorism Center.

Acknowledging the inquiry at his news conference, Obama said: “We want to go back and we want to review every step that was taken. . . . We want to see, is there in fact additional protocols and procedures that could be put in place that would further improve and enhance our ability to detect a potential attack?”

A spokesman for Director of National Intelligence James R. Clapper Jr. said that, while Clapper has instructed U.S. spy agencies to cooperate with the inspector general probe, he does not believe any mistakes were made.

“Director Clapper believes that every agency involved in collecting and sharing information prior to the attack took all the appropriate steps,” said the spokesman, Shawn Turner. “He also believes that it is prudent and appropriate for there to be an independent review of those steps to ensure that nothing was missed.”

Meanwhile, an association of prosthetic companies pledged Tuesday to ensure that people who required amputations as a result of the Boston Marathon bombing would be provided artificial limbs, even if they have no health insurance or inadequate health insurance.

At a news conference, officials from the American Orthotic and Prosthetic Association estimated that 20 to 25 people injured in the bombings will need new limbs and that about half would not have sufficient insurance to cover the prostheses and the care and training needed to use them. They said a variety of companies in the field will donate their services to ensure that victims are able to walk and run again.