A red-and-white “X” was plastered on plywood boards covering the garage door of a Kenduskeag Avenue home on Thursday, as part of a recent push to label buildings that the city deems dangerous for firefighters and emergency responders to enter.

California-based DLJ Mortgage Capital is in the process of foreclosing on the building, which caught fire more than four years ago and has since sat vacant, charred, and neglected. That property is among 20 marked with the metal “X” sign, warning emergency responders of a variety of problems, including failing foundations and structurally deficient roofs.

“Abandoned buildings are always a concern. They are vacant for a reason,” said Fire Chief Tom Higgins. “Structurally they are not safe and yet we need to respond to these buildings for different events.”

The signs mark the city’s latest effort to address safety issues since noting a rise in the number of vacant buildings after the 2008 economic collapse that forced many property owners to fall behind on upkeep.

In August 2013, the city marked 29 buildings as unfit for human habitation — a number that has since ballooned to 141, according to code enforcement statistics. That spike is due to the city doing a better job of identifying such uninhabitable properties, said Code Enforcement Officer Jeremy Martin.

Twenty buildings are deemed hazardous or structurally unstable, and another 29 are deteriorating and have been identified as potential problems, according to a city database from May 2. Vacant buildings are sometimes frequented by squatters and vandals, posing fire or medical risks, to which emergency personnel would need to respond, according to Martin.

The city started cracking down on abandoned homes in 2013, passing a law forcing property owners who leave a building empty for more than 60 days to register with the city. Code enforcement also started hounding property owners and banks to secure doors and windows, Martin said.

Fire and code enforcement officials decided to start marking buildings with the red-and-white signs over the winter — following a model used by other cities and towns across the country, including Massachusetts and New York, according to Higgins.

The decision was not in response to any particular injury or accident at one of the properties, but was an effort to address a general concern shared by members of the fire department, he said.

The city’s code enforcement office has been keeping track of such problem properties over the years, and this winter code and fire officials toured buildings that were considered potentially hazardous.

When police officers, firefighters or medical responders need to enter a building with an “X” sign, a supervisor will call dispatch, Higgins said. Dispatch will then consult the database and tell that supervisor where the hazards may be.

The supervisor will use that information to find a way to respond to the emergency in a safer manner, such as not going into that section when battling a fire, or entering the building from a more stable entry point when rescuing a person inside.