BOSTON—Law enforcement officials said Tuesday evening that they have no concrete information on who planted the two bombs that left more than 170 people injured and three dead near the finish line of the Boston Marathon Monday.

Seventeen of the injured are in critical condition in area hospitals, officials said, with several of them recovering from amputations.

Gov. Deval Patrick announced that President Barack Obama will attend an interfaith ceremony at Boston's Cathedral of the Holy Cross in honor of the victims on Thursday.

FBI Special Agent in Charge Richard DesLauriers pleaded with the public to provide information to help authorities track down the culprit, particularly those who might know someone who was researching how to make bombs, or who saw a suspicious person carrying a heavy black nylon bag near the finish line.

"Someone knows who did this," DesLauriers said. The agent said the investigation in its "infancy," and that authorities do not know who is responsible for planting the bombs. He described the case as "wide open."

DesLauriers said it appeared at least one of the bombs was made of a pressure cooker loaded with nails and metal BBs, though the debris from the scene will have to be examined before they can make that determination. Doctors said metal bearings had embedded into the skin of their patients.

Officials also asked the public to turn in any photographic or digital evidence they have from the scene. They directed people with information to call 1-800-494-TIPS.

The injuries from the explosions include dismemberment and local hospitals say they are treating shrapnel wounds, open fractures and limb injuries. At Massachusetts General Hospital, doctors performed four amputations, and two more patients are at risk for needing amputations. An 8-year-old boy from Dorchester is one of the three known dead, and several of the injured are also children.

So far, two of three victims have been identified: 8-year-old Martin Richard of nearby Dorchester and 29-year-old Krystle Campbell of Medford.

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Boston University announced late Tuesday that a third victim is a Boston University graduate student, who was watching the race with two friends. The Chinese consulate in New York told The Associated Press the victim is a Chinese national.

The university said it would not releasing the victim's name until it has permission to do so from the family.

According to Rep. Stephen Lynch, who has known the Richard family for 25 years, Richard was at the race with several members of his family, including his father, Bill, who had run the race in the past but had skipped the marathon this year because of an injury.

The family had gone to get ice cream and had returned to watch runners along Boylston Street when they witnessed the first blast, according to Lynch. He said the family immediately tried to move off the sidewalk into the street, in an attempt to get away from buildings out of fear of another blast. That’s when the second bomb struck, killing Richard and severely injuring his mother, Denise, and his six-year-old sister, Jane.

Campbell's father, William Campbell, said Krystle was a "wonderful, wonderful girl" who had gone to the finish line to watch her friend's boyfriend run the race Monday.

"This was a cowardly and heinous act," Obama said Tuesday morning in a brief address. He said the bombs are being investigated as an act of terrorism, but emphasized that authorities do not know who is responsible for it. "We will find whoever harmed our citizens and we will bring them to justice," he said.

The president ordered that all flags at public buildings be flown at half staff.

On Tuesday morning, Boylston Street remained closed, but Boston officials reopened some of the perimeter around the site of the explosions. Boston Police Commissioner Ed Davis called the area "the most complex crime scene we've dealt with in the history of our department." A 12-block area remains closed to the public.

There were still signs of the chaos that blanketed the Copley Square area after the explosions went off at 2:50 p.m. Along Huntington Avenue, a stretch packed with hotels where many Boston Marathon runners stayed Monday night, dozens of SWAT vehicles were positioned in spaces where tourists usually board the city’s famous Duck Boat tours. And the side streets leading to Boylston were cordoned off with police tape, as investigators waved off bystanders trying to take photos of the scene.

The shopping mall at the Prudential Center, one of the city’s busiest tourist spots, was reopened but was eerily empty—without the usual stream of workers who use the mall to commute to their offices at the Prudential Tower, one of the city’s tallest buildings.

Along side streets, runners still dressed in their blue and yellow Boston Marathon jackets wandered the streets—some with their suitcases, as they tried to figure out a way to get to the airport, others trying to get in a daily run.

“You’re supposed to keep moving after running a marathon,” said Kathi Russo, a runner from Salisbury, N.C., who had crossed the finish line about 20 minutes before the first blast went off.

Russo, who was running her sixth Boston Marathon, spent hours Monday night trying to get back to her hotel, which was about two blocks from the second bombing site. She described a scene of “chaos” as hundreds of runners were pushed away from the blast site, not quite aware of what had happened.

Russo’s friend, Dianne Allen, was running in a later wave about half a mile away from the site when officials began to stop runners. Allen said people had no idea what happened until word of the bombings began trickling through the crowd, thanks to the few runners who had been carrying their cell phones.

It took Allen several hours to get back to their hotel, where she and Russo were reunited. They said several members of the group they were traveling with had been standing near the finish line and were injured—including a 16-year-old girl, who had a broken fibula, and a husband and wife, who suffered broken legs and burns.

“We don’t know a whole lot more about that,” Allen said. “It’s been hard to get information.”

The race draws many runners from overseas—potentially part of its appeal as a target. Some foreign consulates in Boston urged visitors from abroad to reach out to their families to let them know whether they were safe. Some also updated their social media—notably their Twitter feeds—with the latest from the investigation and useful telephone numbers or other resources.

--Liz Goodwin is reporting from New York.