[Update, 5:35 p.m. ET]
Google has established a person-finder related to the Boston bombings. People who are looking for someone or have information about someone can make reports there.
[Update, 5:31 p.m. ET]
Boston police now appear to be backing away from their commissioner's earlier statement that a third incident -- at the JFK Library 5 miles from the finish line -- might have been related to the Boston Marathon blasts.
On Twitter, Boston police say: "Update JFK incident appears to be fire related."
[Update, 5:21 p.m. ET]
Precautions are being taken at the White House because of the Boston explosions, CNN's Jessica Yellin reports.
[Update, 5:17 p.m. ET]
In the video below, a man describes the initial blast, saying the impact was so strong it "almost blew my head off." He was not injured, but saw many people sustain horrific injuries.
[Update, 5:15 p.m. ET]
The Boston Globe is reporting a much higher injury count. They report that more than 100 people are being treated for injuries, citing local hospitals.
[Update, 5:10 p.m. ET]
Hospitals now say they are treating as many as 51 wounded after the bombings. Two people have been killed, according to Boston police.
[Update, 5:09 p.m. ET]
It will take a long time to clear the area, because lots of people dropped bags and whatever else they had when the finish-line blasts happened. Authorities have to check all of those bags, and bomb squads "may be blowing things up over the next few hours" out of precaution, Boston Police Commissioner Ed Davis said.
Thousands of runners still had yet to finish the race when the bombs exploded in a spectator area along Boylston Street near the finish line, CNN executive producer Matt Frucci at the scene.
[Update, 4:58 p.m. ET]
New details from Boston Police Commissioner Ed Davis:
-- A third explosion happened at the John F. Kennedy Presidential Library "about a half-hour ago." The library is about 5 miles southeast of the Boston Marathon finish line.
-- Police don't immediately know whether that explosion is related to the two near the Boston Marathon finish line.
-- The two blasts near the finish line -- along Boylston Street near Copley Square -- "happened 50 to 100 yards apart."
-- "We're recommending to people that they stay home ... and that they don't go anyplace and congregate in large crowds."
-- Relatives of people who may be missing in the area can call the mayor's hotline at 617-635-4500.
-- Anyone who has information about the bombings or saw anything suspicious can call 1-800-494-TIPS.
[Update, 4:46 p.m. ET]
Massachusetts Gov. Deval Patrick says "this is a horrific day in Boston."
"My thoughts and prayers are with those who have been injured," Patrick said in a statement released this afternoon. "I have been in touch with the president, Mayor Menino and our public safety leaders. Our focus is on making sure that the area around Copley Square is safe and secured. I am asking everyone to stay away from Copley Square and let the first responders do their jobs."
[Update, 4:45 p.m. ET]
It appears that so many people are using cell phones in the center of Boston, consistent service is hard to get -- and the overload is hampering the investigation, two federal law enforcement sources tell CNN.
[Update, 4:40 p.m. ET]
Another journalist says she saw victims who lost limbs. This account is from Boston Globe political reporter Cynthia Needham:
"Outside MGH: Head of emergency medicine says 19 have been brought to MGH, six critically injured, some with amputations," she posted to Twitter.
Earlier, we noted that Boston.com sports producer Steve Silva reported that he "saw dismemberment" and "blood everywhere."
[Update, 4:37 p.m. ET]
Organizers with the London Marathon, scheduled for this coming Sunday, have taken notice.
"We are deeply saddened and shocked by the news from Boston," London Marathon officials said Monday. "Our immediate thoughts are with the people there and their families. It is a very sad day for athletics and for our friends and colleagues in marathon running. Our security plan is developed jointly with the Metropolitan Police and we were in contact with them as soon as we heard the news."
[Update, 4:30 p.m. ET]
Boston firefighters have found what they believe is an unexploded device after the blasts, a government official said, according to CNN's Joe Johns.
[Update, 4:27 p.m. ET]
"I saw blood everywhere," Boston.com sports producer Steve Silva told Boston.com.