(Note at 7:50 a.m. ET, April 18: We've begun a new post to track Thursday's developments.)
Investigators made progress Wednesday, as they tried to determine who planted two bombs near the finish line of the Boston Marathon, leaving three people dead and injuring about 180.
NPR's Tom Gjelten reported the big news: A senior law enforcement official told him that investigators have video of a man setting down a bag and leaving the scene. Despite conflicting reports, it turned out that the FBI had not made an arrest.
There was also good news: The New York Times reported that hospital officials said the 49 victims still at Boston-area hospitals would likely survive their injuries.
We'll update this post as news happens. See this note about how we cover events such as this.
9:19 p.m. ET. Where Things Stand.
INVESTIGATION: There were conflicting reports in the early afternoon about whether a suspect was or was not in custody. As of 2:15 p.m. ET, sources with knowledge of the investigation were telling NPR's Tom Gjelten that an arrest had not yet been made.
Law enforcement is expected to brief reporters later tonight.
Tom reports that a senior law enforcement official told him that authorities have reviewed video of a man setting down a bag and leaving the scene, but that does not necessarily make him a suspect. "We need more than that," the official said.
CNN's Deb Feyerick said what they know is the FBI has a picture of a suspect. What's unknown is whether they've even identified the suspect.
NPR's Dina Temple-Raston reports that an official familiar with the investigation told her the bomb was "a pressure cooker design with nails and ball bearings inside." And Dina said on Morning Edition Wednesday that FBI investigators thought the key clue to finding who's responsible would come from a photo or video taken by a spectator — evidence that "people don't know is important, but the FBI looks at and sees some clue." At the website of the FBI's Boston Division, officials posted this appeal for help: "If you have any information that could be of assistance, please call 1-800-CALL-FBI (prompt #3). No detail is too small."
DEATHS, INJURIES: The three people who were killed have been identified: Martin Richard was 8 years old and from suburban Dorchester. Krystle Campbell was 29 and lived in Medford, Mass. Lingzi Lu of China was a graduate student at Boston University.
About 180 people were injured. The New York Times reports that 49 victims remain in area hospitals. Officials told the paper all of them are expected to survive their injuries.
Update at 7:17 p.m. ET. Briefing Cancelled:
The scheduled 8 p.m. ET. briefing has been cancelled, the Boston Police Department just tweeted.
Update at 7:17 p.m. ET. New Briefing Time:
The Boston Police Department just tweeted that the FBI will "make a brief statement" at 8 p.m. We'll bring you whatever news comes of that, as it happens.
Update at 7:12 p.m. ET. Likely All Victims At Hospital Will Survive:
The New York Times reports that hospital officials say the 49 victims still at Boston-area hospitals would likely survive their injuries.
At one point, more than 100 people were at area hospitals, so this is good news. The Times reports that 12 victims remain in critical condition.
Update at 4:31 p.m. ET. Press Briefing Postponed:
A briefing scheduled for 5 p.m. ET. has been postponed, the Boston Globe reports.
Update at 4:15 p.m. ET. Courthouse Evacuated, All Clear Given:
The John Joseph Moakley Federal courthouse in Boston was evacuated about an hour ago.
WBUR reports that the evacuation occured "because of a received bomb threat."
At about 4:15, a man waving a green flag gave the all-clear. Adam Gabbatt, of The Guardian, tweeted that a court house official said: "We had a bomb threat; the building was clear; it's being reopened for staff only."
One other thing to keep in mind, authorities are scheduled to brief reporters at 5 p.m. ET. We will live blog that press conference.
Update at 2:48 p.m. ET. Justice Says No Arrest:
The Boston Globe, which earlier reported an arrest had been made, is now walking back its report, saying the U.S. Attorney's office says no suspect has been arrested.
The AP, which also reported the arrest, has also walked back the report, saying "federal officials deny that Boston Marathon bombing suspect is in custody."
As we told you earlier:
"CNN and the AP had been reporting that there a suspect in custody — or soon would be. CNN later reversed course, however, saying that additional sources were saying no one had been taken into custody. Meanwhile, The Boston Globe was saying "an arrest is imminent." CNN later backtracked, though. And at 2:34 p.m. ET the Boston Police set the record straight, tweeting that 'Despite reports to the contrary there has not been an arrest in the Marathon attack.'"
Update at 2:40 p.m. ET. Boston Police Say There's Been No Arrest.
"Despite reports to the contrary there has not been an arrest in the Marathon attack," according to a statement just posted on the Boston Police Department's official Twitter page.
Update at 2:32 p.m. ET. CNN Backtracks; Says No Arrest Yet:
Three senior federal officials have now told her that there has not yet been an arrest, CNN contributor and former Bush administration national security adviser Fran Townsend just said on the air. Earlier, she was reporting her sources were telling her than an arrest had been made.
Update at 2:15 p.m. ET. No Arrests Yet, NPR Is Told:
NPR's Tom Gjelten reports that his sources say there have not yet been any arrests made. As we've noted, though, both AP and CNN are saying that their sources tell them an arrest has been made.
Sources also tell NPR that there is video evidence showing a person who appears to have placed a bag at the scene that may have contained one of the bombs.
Update at 2:05 p.m. ET. Suspect In Custody, AP Reports:
The wire service just moved this "alert":
"Law enforcement official: Boston Marathon bomb suspect in custody, expected in federal court."
"A law enforcement official briefed on the investigation says a suspect in the Boston Marathon bombings is about to be arrested," The Associated Press reports.
CNN says an arrest has already been made. It's getting that information from Fran Townsend, CNN contributor and former National Security adviser to President George W. Bush, who says that's what she has been told by a federal law enforcement source in a position to know. It's also getting word of an arrest from one of correspondent John King's "Boston law enforcement sources." NPR is working to get independent confirmation.
CNN is reporting that "investigators believe they have identified a suspect in the Boston Marathon bombings, a source who has been briefed on the investigation told CNN's John King exclusively." They told CNN that videos taken at the scene helped them zero in on the individual. The Boston Globe says it has been told by a source that "authorities have an image of a suspect carrying, and perhaps dropping, a black bag at the second bombing scene." Authorities are set to brief reporters at 5 p.m. ET.
Boston University now writes that the third person killed was "Lingzi Lu ... a graduate student in mathematics and statistics." The New Yorker reports that she "was a native of Shenyang, a city in northeastern China about a hundred miles from the North Korean border. She attended Shengyang Northeastern High School, from which she graduated in 2008. She did well enough there to land a spot at the Beijing Institute of Technology, where she pursued a bachelor's degree in international economics. In Boston, she had hoped to obtain her master's. On Monday afternoon, Lu joined the marathon-watchers with two other overseas Chinese friends, partaking in one of their first and more cacophonous of American traditions."
Update at 12:35 p.m. ET. "Significant" Progress?
CNN is reporting it has been told by authorities that "significant" progress has been made today on the investigation.
Update at 10:20 a.m. ET. Comparison To Vancouver Riot Investigation:
Following the riot in Vancouver, Canada, two years ago when the NHL's Canucks lost the Stanley Cup championship, authorities requested photos and videos taken by the public to help find those who had set fires and thrown gasoline bombs. On WBUR this morning, correspondent David Boeri said more than 1 million photos were submitted and "1,200 to 1,600 hours of video." In large part because of that evidence, police "were able to determine who had thrown bombs" and were responsible for other misdeeds.
Boeri said authorities in Boston are hoping the huge amount of evidence that's likely in photos and videos taken by spectators and the media will help solve the marathon bombings.
Reminder: At the website of the FBI's Boston Division, officials have posted this appeal for help: "If you have any information that could be of assistance, please call 1-800-CALL-FBI (prompt #3). No detail is too small."
Most of the 10 or 11 victims being treated at Boston Medical Center who were initially in critical condition have made good progress, chief trauma surgeon Dr. Peter Burke just told reporters. He said only two patients remain in critical condition, and he expects them to survive their injuries. Burke added that 10 patients at the hospital are now considered to be in "serious" condition. An additional seven are said to be in "fair" condition.
No "specific suspect or targets" have yet been identified, Massachusetts Gov. Deval Patrick (D) said on Boston's WBUR this morning. And while "every hour, every day" brings investigators "a little closer" to finding out who was responsible, Patrick said everyone needs to "settle down and settle in for a long, painstaking investigation."
"This is the kind of investigation that requires picking up tiny pieces [of evidence] and scouring blocks ... a square inch at a time," the governor added.
Update at 8:05 a.m. ET. Pressure Cooker Lid Found?
CNN is reporting that the lid of a pressure cooker has been found on the roof of a building near the site of Monday's explosions.
Update at 8 a.m. ET: 100 Of The Injured Released, CNN Reports:
The latest estimate on how many people were injured, CNN says, is 183. And of those, about 100 are out of the hospital, the news network adds.
-- NPR's Coverage.
-- WBUR's coverage.
Note: As happens when stories such as this are developing, there will likely be reports that turn out to be mistaken. Monday, for example, authorities at one point said they thought there also had been an explosion at the JFK Library in Boston. But it turned out there had been a fire, not an explosion, and there's no known link at this time to the marathon attacks.
We will focus on news being reported by NPR, other news outlets with expertise, and statements from authorities who are in a position to know what's going on. And if some of that information turns out to be wrong, we'll update.