A group of heavily armed cadre of Garo National Liberation Army (GNLA) fired indiscriminately in a small market near Williamnagar district headquarters at about 7 pm, the police said.
The incident came in the wake of police intensifying operations against the militants in the jungles, sources said.
In a separate incident, militants shot dead two persons who were allegedly extorting money in the district.
The bodies were found from the side of a road with a note left near the body, the police said.
Last Friday, an assistant teacher at the MP Secondary School, Resubelpara, was shot dead on suspicion that he was a police informer.
The outfit, which is led by a police deserter, Champion R Sangma, had also carried out a number of kidnappings and extortions from government officials and businessmen in the area besides killing security personnel and labourers, in the three districts of Garo hills and including West Khasi Hills district, sources said.