Federal investigators learned about the conversation during an interrogation of Dzhokhar Tsarnaev in the hospital that began Sunday night and went into Monday morning, officials said. The information didn't reach the NYPD until Wednesday night.
Kelly said he told the FBI that the delay was troubling.
"We did express our concerns over the lag," Kelly said following a promotions ceremony.
Kelly and Mayor Michael Bloomberg announced the findings Thursday during a City Hall news conference.
"We thought it was important to put that information out because the people of New York City deserve to know that," Kelly said.
Tsarnaev and his brother had been identified as suspects and were the target of a massive manhunt when, by his account, they discussed what to do with the remainder of the explosives used in the marathon attack, Kelly said. They came up with the idea of striking in Times Square, but Tsarnaev's older brother was killed and he was captured before they could leave the area and do any more harm, New York City officials said.
Still, Rep. Peter King of New York has said the information was important because "there could be other conspirators out there and the city should have been alerted so it could go into its defensive mode."
Asked what difference it would have made to have learned about the conversation sooner, Kelly responded: "I don't want to speculate. The fact of the matter was there was a 48-hour lag."
The FBI declined to comment on Friday.
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