NEW YORK — A bomb explosion last Thursday at a Kmart store in Neptune Beach, Fla., is still under investigation by the Federal Bureau of Alcohol, Tobacco and Firearms, according to Kmart officials.

David Easterbrook, district manager for Kmart’s North Florida and Southern Georgia stores, said Sunday that the bomb exploded at 1:34 p.m. as the store’s personnel and training manager was investigating a plume of smoke rising from a 13-gallon plastic trash can on the sales floor. Kmart had not received any threats, he said.

The manager was in “fair” condition at a local hospital Sunday with chest and leg injuries. Three other workers were treated for minor injuries and released from the hospital the day of the explosion. No customers were hurt and there was no extensive damage to the store, Easterbrook said.

As a precautionary measure, the Neptune Beach store and four other Kmarts in the East Jacksonville area were closed Thursday for the rest of the day. All reopened for business Friday after investigations were conducted in each of the stores to assure customer and employee safety, Easterbrook said.

Business was not substantially affected by the incident said Teri Kula, manager of corporate communications for Kmart Corp. in Troy, Michigan.