[Updated at 7:35 p.m. ET]

BOSTON—No arrests have been made, but investigators are gaining ground on identifying the assailant behind the fatal bombing at the Boston Marathon, Massachusetts Gov. Deval Patrick said late Wednesday.

“They are making progress, there’s no doubt about it,” Patrick told reporters.

The governor asked for the public’s patience as the probe continues into who planted the two shrapnel-filled bombs that exploded near the marathon finish line on Monday. Three people died in the blast, and at last 170 were injured.

“I take comfort in the fact that the investigation is as thorough as it is,” Patrick said. “When you consider the size of the crime scene and the fact that they have to go through it square inch by square inch, that’s exactly what we want and what we need.”

The FBI had scheduled a news conference for 8 p.m. Wednesday, but minutes later canceled the briefing without explanation.

Earlier Wednesday law enforcement authorities forcefully denied a flurry of news reports that an arrest had been made in the case.

Several news organizations including The Associated Press, CNN and the Boston Globe, reported that a suspect had been arrested and was en route to the Moakley Federal Courthouse in South Boston. The Boston Police Department and the U.S. attorney's office in Boston denied those reports on Wednesday afternoon, saying no arrest had been made.

The FBI issued a sternly worded rebuke to the press for reporting that a suspect was in custody.

"Contrary to widespread reporting, no arrest has been made in connection with the Boston Marathon attack," Special Agent Greg Comcowich said in the statement. "Over the past day and a half, there have been a number of press reports based on information from unofficial sources that has been inaccurate." He added that the reports cause "unintended consequences," and that the media should verify information through "official channels."

The Associated Press said its original unnamed source stands by the information.

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Meanwhile, the Moakley Federal Courthouse was evacuated around 3 p.m. Wednesday after a bomb threat. Hundreds of reporters and court employees—some of whom had gathered out front to see if a suspect would be brought there—calmly evacuated the building and walked across the street. More than a dozen news vans with satellite uplinks lined the street adjacent to the courthouse while news helicopters hovered above.

About 90 minutes after the building was evacuated, a maintenance worker emerged from the courthouse waving a green flag signaling "all clear," and people were allowed back into the building.

A small plaza at the Brigham & Women's Hospital was also evacuated due to an abandoned vehicle in the area, a spokeswoman confirmed.

Authorities combed through video footage to find an image of "a suspect carrying, and perhaps dropping, a black bag at the second bombing scene," The Boston Globe reported. CBS News reported that the video showed a man in a black jacket talking on his cell phone "placing a black bag at the second bomb site outside of the Forum restaurant on Boylston Street and then leaving the area before that explosion." Police used the time stamp on the video to scan all the calls made in the area to try to track him down, CBS News' Bob Orr reported.

The FBI was scheduled to hold a press conference Wednesday at 5 p.m., but announced it would be postponed until later Wednesday night due to the courthouse bomb threat. The breakthrough in the investigation came from analyzing department store surveillance video and video from a news station.

An FBI spokesman in Boston and a spokeswoman for Boston Mayor Tom Menino declined to comment on the reports to Yahoo News.

Eight-year-old Martin Richard, 29-year-old Krystle Campbell and 23-year-old Lingzi Lu were the three people killed in the explosion. Several of the injured needed amputations.

Police have said the bombs were fashioned from debris-filled pressure cookers and stuffed in black bags. They were located about 100 yards apart.

A Boston-area federal agent told Yahoo News that he suspects there will be more than one arrest in the case eventually.

“To carry one heavy bag in is one thing, but a single suspect having two bags would have stood out,” the law enforcement source told Yahoo News.

—Liz Goodwin reported from New York. Holly Bailey and Dylan Stableford and also contributed reporting from Boston.