A comprehensive immigration bill introduced last week in the Senate also may undergo changes in response to Boston. One of its authors, Sen. Lindsey Graham, R-S.C., has suggested strengthening background checks done on certain immigrants considered higher-risk, such as refugees or asylum-seekers.
In New York, Kelly and Bloomberg said they were briefed on the New York plot on Wednesday night by the task force investigating the Boston bombing.
Rep. Peter King, R-N.Y., said in a CNN interview that the city should have been told earlier "so it could go into its defensive mode."
Kelly, citing the interrogations, said the Tsarnaev brothers "planned to travel to Manhattan to detonate their remaining explosives in Times Square" four days after the Boston bombing.
A day earlier, Kelly said that Dzhokhar Tsarnaev had talked about coming to New York "to party" after the attack and that there wasn't evidence of a plot against the city. But Kelly said a later interview with the suspect turned up the information.
Kelly said there was no evidence New York was still a target. But in a show of force, police cruisers with blinking red lights were lined up in the middle of Times Square on Thursday afternoon, and uniformed officers stood shoulder to shoulder.
Outside Penn Station, Wayne Harris, a schoolteacher from Queens, said: "We don't know when a terrorist attack will happen next in New York, but it will happen. It didn't happen this time, by the grace of God. God protected us this time."
In 2010, Times Square was targeted with a car bomb that never went off. Pakistani immigrant Faisal Shahzad had planted a bomb in an SUV, but street vendors noticed smoke and it was disabled. Shahzad was arrested as he tried to leave the country and was sentenced to life in prison.