(CNN)Explosions and fires rocked nearly 40 homes and businesses in three Massachusetts towns on Thursday, leaving at least six people injured, including two in critical condition. The incidents may have resulted from an issue with gas service, officials said.
Explosions set structures on fire across dozens of blocks and forced evacuations in Lawrence, Andover and North Andover, Massachusetts State Police said. Not all gas had been shut off by Thursday evening and some of the lines were overpressurized, officials said.
"This has been an overwhelming event," Andover Fire Rescue Chief Michael B. Mansfield told reporters.
Witnesses described a harrowing scene. Lawrence resident Ra Nam told CNN affiliate WCVB that a boiler in his house caught fire. Minutes later, a loud boom came from his neighbor's home. Three people rushed out.
Firefighters put out 35 fires in Andover, town officials said. The cities are north of Boston.
"Incidents are across a wide swath of dozens of blocks across Lawrence and North Andover," state police said via Twitter. There were at least 70 responses to fire, explosions and gas odor reports.
Officials were shutting down power in Lawrence, North Andover and Andover because of the emergency, state police said on Thursday evening. About 18,000 customers were without power.
Lawrence General Hospital received six patients, two of whom were in critical condition, according to spokeswoman Jill Halsey. Andover officials said a firefighter was among the injured.
Lawrence Councilor Marc Laplante said residents of South Lawrence also are being evacuated.
A middle school in North Andover and a senior center in Andover were set up for evacuees, according to the agency. Andover officials asked residents and businesses in gas-serviced neighborhoods to evacuate. Schools are closed Friday in Lawrence and Andover.
Residence halls and academic buildings at Merrimack College were evacuated, according to Jim Chiavelli, chief of staff for the college president. Students were waiting outside buildings for the gas to be shut off.
Lawrence Mayor Dan Rivera estimated there are between 20 and 25 fires across the city, according to WCVB.
Residents in the area who have service from Columbia Gas "should evacuate their homes immediately," state police said.
Gas lines were being depressurized by the company. The state police said some neighborhoods were evacuated because of gas odors.
There was no immediate information available about the extent of the damage. "Far too early to speculate on cause" of the explosions, authorities said.
Gas service was interrupted for 250 customers in Lawrence last month "due to hit line by a third party," according to Columbia Gas.
The National Transportation Safety Board is gathering information.
CNN's Lauren del Valle, Phil Gast and Taylor Romine contributed to this report.