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Faulty installation of a natural gas water heater by a Hopkins homeowner led to an explosion and fire that killed him and his wife in the house he built more than a half-century ago, officials said Wednesday.

The incident on July 27 at the home in the 200 block of 21st Avenue N. claimed the lives of Hubert "Herb" Vassar, 85, and Sharon Vassar, 83. The blast shook nearby homes and could be heard 15 blocks away.

Officials with the State Fire Marshal's Office and the Hopkins Fire Department "learned that the residence's water heater was recently replaced, and a gas line was not reattached following the installation," a statement from the Fire Department read. "This allowed gas to leak into the residence and eventually ignite."

Fire Chief Dale Specken told the Star Tribune that Herb Vassar "may have pulled out the gas dryer to get to and replace the water heater" in the basement of the home he built in 1961.

"We then believe [he] replaced the water heater and put the dryer back in place and failed to attach the line to the dryer and turned on the gas to the water heater and dryer," the chief continued, "filling the basement full of gas and causing the explosion."

Specken said Herb Vassar did not get a city-issued permit as required for such a task, "but it is not uncommon for someone installing their own to not get a permit."

The Fire Department said investigators have yet to pinpoint what ignited the natural gas, but sources vary from flipping a light switch to static electricity to pilot lights. Fire officials classified the blaze as accidental in nature.

Scott Waryan, who grew up in the neighborhood, said Herb Vassar kept the property in nice shape and was often working on projects.

David Viland, 73, said on the day of the fire that he was at home down the street when he heard the boom. It shook his house, he said, reminding him of bombs or heavy artillery from his time serving in Vietnam.

He walked over to see his neighbor's house, which was engulfed in flames. Within 15 minutes, it had collapsed.

"Standing out on the street here, I felt the heat on my face," Viland said.

One witness to the explosion was a package delivery person, who said the blast "reminded him of being in a war zone," according to a police report released Wednesday.

The 52-year-old man got on top of the garage and thought he could hear a voice nearby, the report said.

A neighbor joined him, and they both started calling Herb Vassar's name.

The delivery worker "was very shaken and upset," detective Eliana Welbes wrote in the report. "I reassured him that he did everything he could to the best of his abilities."

The Vassars were married for nearly 64 years and raised five children in the home "where they kept a meticulous lawn and garden," according to their online obituary. Survivors include 11 grandchildren.

Herb Vassar "was a carpenter by trade and a proud union member throughout his career," the obituary read.

Sharon Vassar was a secretary for the U.S. Fish and Wildlife Service, "and was very proud of her work and contributions to saving endangered species," the obituary noted.

Services for the Vassars are scheduled for 1 p.m. Friday at St. Gabriel the Archangel Catholic Church, St. Joseph Campus, 1310 Mainstreet, Hopkins, with a graveside service to follow at St. Margaret's Cemetery in Minnetonka. Visitation is 4-8 p.m. Thursday at the Washburn-McReavy Hopkins Chapel, 1400 Mainstreet.