CHISINAU, October 15 (RIA Novosti) - The day after a devastating grenade attack at a concert in Moldova's capital, a grenade-armed man is now threatening to blow himself up outside the General Prosecutor's Office, officials said.

At least 40 people were injured late on Wednesday night when a grenade went off during a concert being held on Chisinau's main square as part of city day celebrations.

A spokesman for the Interior Ministry told RIA Novosti that the man standing outside the prosecutor's office is demanding the release of his son, who was sentenced seven years ago for murder.

Psychologists are negotiating with the man, and security forces have been deployed nearby.

A press briefing on Wednesday's explosion was scheduled to be held outside the prosecutor's office.

Moldovan Interior Minister Viktor Katan earlier said the causes of the concert blast remain unknown.

"It remains to be seen whether this is a result of hooliganism, negligence or other causes. We are not ruling out the version of a terrorist attack," he said.