The gunman who went on a shooting rampage in a popular gay nightclub here shot nearly all of his victims in the first stages of the assault, then was utterly “cool and calm” while he talked by phone to law enforcement officials about further carnage, claimed allegiance to the Islamic State and praised the Boston Marathon bombers, officials said on Monday.

As officials offered new details about the worst mass shooting in American history, which left 49 people dead and 53 wounded, Chief John Mina of the Orlando Police Department said that the gunman, Omar Mateen, 29, told police negotiators — falsely, they later discovered — that he had explosives and accomplices at Pulse nightclub.

Mr. Mateen eventually made comments persuading them that “there would be an imminent loss of life,” Chief Mina said, prompting the chief to end a three-hour standoff and order the assault that killed Mr. Mateen and freed dozens of people trapped in the club.

President Obama, speaking to reporters at the White House, said it appeared the attack was “an example of the kind of homegrown extremism that all of us have been concerned about,” with a gunman who was inspired by radical material he found online.

Even as American military forces and intelligence agencies attack the Islamic State and other extremist groups, Mr. Obama said, “one of the biggest challenges we are going to have is this kind of propaganda and perversions of Islam that you see generated on the internet.”

New information also emerged Monday on Mr. Mateen, including frightening statements he made three years ago to co-workers at a local courthouse about being tied to terrorism, and that the resulting F.B.I. investigation was extensive, lasting 10 months.

“First he claimed family connections to Al Qaeda,” which, like the Islamic State, is a Sunni Muslim terrorist group, James Comey, the F.B.I. director, said Monday. “He also said he was a member of Hezbollah,” a Shiite group in conflict with Al Qaeda and the Islamic State.

“He said he hoped that law enforcement would raid his apartment and assault his wife and child so that he could martyr himself,” Mr. Comey said.

“Our investigation involved introducing confidential sources to him, recording conversations with him, following him, reviewing transactional records from his communications,” and searching government records for mentions of him, the director said. “We then interviewed him twice. He admitted making the statements his co-workers reported, but explained that he did it in anger because he thought his co-workers were discriminating against him and teasing him because he was Muslim.”

Mr. Mateen had a chilling history that included talking about killing people, beating his former wife and voicing hatred of minorities, gays and Jews; most of his victims were gay, Latino, or both. His father, Seddique Mir Mateen, an Afghan immigrant, has said that he was particularly enraged by seeing a same-sex couple kiss, though the elder Mr. Mateen said in a video he posted online that he believed that it was up to God to punish gays. But on Monday, the father, teary-eyed, told reporters that he had no inkling of what was to come.

“If I knew 1 percent about what he was doing, I would have called the F.B.I.,” the elder Mr. Mateen said. “He went against my principles as a father and as a U.S. citizen.”

Chief Mina said that an off-duty officer who had been working at Pulse responded to shots fired at about 2 a.m. Sunday. Additional officers rushed to the scene, he said, and entered the nightclub, where they engaged in a gun battle with Mr. Mateen, forcing him to retreat to a bathroom where officers believed he had four to five hostages. About 15 to 20 people were in another bathroom.

“At that time we were able to save and rescue dozens and dozens of people and get them out of the club,” Chief Mina said. A SWAT team was called and took up positions in a bathroom across from where Mr. Mateen had taken cover.

From that point, “it kind of stabilized and the suspect had barricaded himself in the bathroom,” and there was no shooting during the subsequent standoff, the chief said. Mr. Mateen called 911, beginning a series of calls and conversations with the police, in which he declared allegiance to the Islamic State, the terrorist group that has taken over parts of Syria, Iraq and Libya. The attackers who killed 14 people in San Bernardino, Calif., in December also stated their allegiance to the group.

“He was cool and calm when he was making those phone calls to us,” Chief Mina said. “We had a team of crisis negotiators that did talk to the suspect, just trying to get as much information as possible, and they focused on what they could do to resolve the situation.”

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