A two-alarm fire in North Bethany that occurred on December 11. It's the fourth fire in a string of arsons that have targeted homes under construction in a housing development. Tualatin Valley Fire & Rescue

Firefighters in Washington County battled a two-alarm fire in a North Bethany housing development early Monday morning. The house that burned was vacant and in the final stages of construction.

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At 12:39 a.m., a Washington County Sheriff's Office patrol deputy spotted the fire at 7864 Northwest 169th Ave. and also saw smoke coming from the home next door.

It’s the fourth fire in the neighborhood since August. The first two fires were declared arson, and a third fire in November was labeled as suspicious.

“All of these fires are being evaluated and there are concerns that they may all be intentionally set,” said Chief Cassandra Ulven, with Tualatin Valley Fire and Rescue.

The fire department is working with arson detectives at the Washington County Sheriff’s Office to determine where and how the fire started.

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The three other North Bethany fires were:

The first fire was on Aug. 16 at Northwest 169th & Trillium.

The second fire was a block away on Aug. 27 at Northwest 169th & Madrone.

The third took place on Nov. 21 near the intersection of NW 167th and NW Catalpa St.

“The fires are very similar in terms of time of day, type of homes, fire location,” said Deputy Jeff Talbot, public information officer with the Washington County Sheriff’s Office.

Since August, the sheriff’s office has increased patrols and the developer has also hired private security. Talbot said arson investigators are reviewing security cameras on the property. He’s also asking any neighbors in the area to alert police to any suspicious activity.

Four fires have hit a North Bethany housing development since August. Tualatin Valley Fire & Rescue

“People are understandably worried. We’re worried as well, frankly,” said Talbot. “We’re trying to do as much as we can to capture who is doing this.”

Last month, The Portland Tribune reported that more than 200 people attended a community meeting about the fires. Many nearby residents were concerned about the arson and worried about the safety of their properties. The sheriff's office is still investigating and has not released a possible suspect.

The North Bethany housing development where the four fires have occurred is being built by Noyes Development. The company says the project began in the summer of 2016, with home construction starting earlier this year. So far 18 homes have been completed, with 26 other homes in various stages of construction.