On Nov. 30, two and a half months before the Parkland massacre, an unidentified caller from Massachusetts told the Broward County Sheriff’s Office that Mr. Cruz was collecting guns and knives and that “he could be a school shooter in the making.”

Two years before, the office reported receiving “thirdhand information” from the son of one of Mr. Cruz’s neighbors that he “planned to shoot up the school on Instagram.”

The tip that the F.B.I. received in early January from someone close to Mr. Cruz suggested that he owned a gun and had talked about carrying out a school shooting. But the bureau failed to investigate, even though the tipster said Mr. Cruz had a “desire to kill people, erratic behavior and disturbing social media posts.”

That information should have been sent to the Miami F.B.I. field office, the bureau said.

The F.B.I. also received a tip from a bail bondsman in Mississippi in September about a suspicious comment left on his YouTube channel by a “nikolas cruz” who professed a desire to be a “professional school shooter.” The bondsman notified YouTube, which promptly took down the comment.

The F.B.I. said it did not have enough information to determine if “nikolas cruz” was a real name or a pseudonym, and the bureau said it could not justify keeping a file on the tip open and closed it in October.

When the second tip reached the F.B.I. in West Virginia in January, a specialist was able to view the earlier tip, too. But even then, the specialist, in consultation with her supervisor, decided that there was not enough evidence to pursue it and that it did not appear to be an imminent threat.

The acting F.B.I. deputy director, David L. Bowdich, briefed congressional staff members about the case on Friday and acknowledged the bureau’s failure to investigate, according to three people with direct knowledge of the meeting.