(CNN) -- Belarus investigators described a suspect in Monday's subway bombing that killed 12 people as a 27-year-old man, but they were unsure about motives or if the bomber had help.
"We do not rule out that he had accomplices," Belarusian State Security Committee head Vadim Zaitsev said Tuesday.
Three other men have been held for questioning, Zaitsev said at a news conference.
Preliminary findings suggest the blast was caused by a radio-controlled device, and that no suicide bombers were involved, Belarusian officials said.
The Belarusian KGB is considering three possible motives behind the bombing, including an attempt at destabilizing the security situation in Belarus, revenge by extremist organizations, and an act by a madman, Zaitsev said.
He added that the KGB has not ruled out that the suspect, who was wearing a brown jacket and a brown wool hat, may have been a paid mercenary.
Police are studying closed-circuit TV camera recordings that might shed light on the crime, Belarus Interior Minister Anatoly Kuleshov said.
A train was standing at the Oktyabrskaya metro station in Minsk when a handmade bomb planted under a bench exploded Monday, officials said. The incident is described by officials as an act of terrorism.
The bomb, with estimated power equivalent of 5 kilograms of TNT, was stuffed with shrapnel, nails and small metal balls, the state-run news agency Belta reported, citing security officials.
The death toll rose to 12 when one of the victims died in a hospital early Tuesday, RIA Novosti reported.
"Experienced investigators and criminalists" from Russia's investigative committee were on their way to Minsk to help with the investigation, according to the committee's website.
Zaitsev said Belarus welcomes the international help.
"We will not refuse help from other countries either," he said.
Victims' families will receive $10,000 from the government and their funeral expenses will be covered, Belarus state media reported Tuesday. Funerals are expected to take place Wednesday and Thursday, according to the city hall in Minsk.
More than 150 people have been hospitalized, and some of the wounded are in serious or critical condition, Russian media said.
The government declared there will be a national day of mourning on Wednesday, Belta said.
U.N. Secretary-General Ban Ki-moon issued his condolences. British Minister for Europe David Lidington expressed his shock.
"I understand that the Belarusian authorities believe this was an act of terrorism. The United Kingdom utterly condemns all such actions, and our thoughts are with all of those affected by this tragic event," he said.
Russian President Dmitry Medvedev called the blast a "terrorist attack" and offered to help investigate who was behind the explosion, the Kremlin said in a statement.
Medvedev made the remarks in a telephone call with Belarusian President Alexander Lukashenko, who later said at an emergency government meeting that he accepted the offer of assistance.
Lukashenko ordered officials to increase public transport security, according to the state-run news agency Belta.
He also instructed security officials to investigate whether this latest blast could have any connection with an explosion in downtown Minsk in July 2008, Belta reported. That bomb went off during an open-air concert, injuring some 55 people.
"Perhaps, they are linked. Establish who could benefit in disrupting the calm and stability in the country and find out who is behind the attack," Lukashenko said, according to Belta.
CNN's Maxim Tkachenko contributed to this report