An underground fire that destroyed a Charles Street bookstore, closed several blocks and left thousands without power last year was caused by gases that built up in a manhole before detonating, according to a consultant’s report city officials released Tuesday night.

The consultant, RTI Group, discovered multiple risk factors, including conduits that were overcrowded with equipment and cables from multiple utilities.

Fiber-optic cables and power cables were sharing the same ducts through the conduit, the report found. Equipment “spanning approximately 100 years” was found in the manhole.

The manhole exploded and a fire ensued on Sept. 29, 2024, in the 300 block of North Charles Street near Pleasant Street, causing widespread damage in the neighborhood. There was another manhole fire in the block that January, and there may have been prior damage in the area as well, according to the report.

Photos taken after the January fire that were included in the report showed a tangled mess of wires in the conduit.

“The unacceptable overcrowded conditions, proximity, location and time since repairs in the January event constitute a contributing factor in the September event,” the report states.

In the September event, one early-morning caller to 911 described it as a “big bang,” and photos showed that “the fiber cable and splice box” were ejected out of the manhole. The manhole cover was splintered into five pieces, according to the report.

A couple hours later, a fire broke out in the bookstore in the Brown’s Arcade building due to a lack of “fire stop plugs” in the duct, according to the report. Two more manhole covers were blown out as well.

The report listed a slew of recommendations to prevent future explosions and fires in the city’s underground conduits, including:

• Ensuring fiber-optic cables and equipment are separated from power cables by at least 12 inches
• Improving monitoring
• Developing a citywide conduit risk map identifying vulnerable spots in the system
• Developing emergency plans for firefighters and Baltimore Gas and Electric crews who respond to manhole fires
• Exploring venting options for manholes
• Establishing a utilities safety committee
• Obtaining CitiWatch footage and other videos within 24 hours of manhole fires

“This issue has existed for years,” Mayor Brandon Scott said in releasing the report, “but this is the first time the City has hired a professional engineering firm to actually develop a comprehensive set of recommendations for preventing these fires in the future.”

Veronica McBeth, director of the city transportation department, praised her department’s underground conduit division for doing “an amazing job” managing the system. She pledged to work with utility companies “to develop a plan to mitigate the threat that these manhole explosions and underground fires pose to our residents.”

Underground fires in the aging city conduit system have been an ongoing problem in Baltimore. Hundreds of miles of clay pipes run under the city streets, carrying wires and cables for a variety of utilities and services.

Part of the reason the city hired RTI Group to issue a report was that its own investigative ability was limited. At an October 2024 City Council hearing, Baltimore Fire Chief James Wallace said his investigators weren’t allowed to go underground and look at the conduit.

“We do not have the ability to place a fire investigator underground,” Wallace said then, citing training requirements and laws that limit what the department can do.

The city did not hire RTI Group to investigate until another manhole caught fire last June.

Before RTI Group’s report was made public, both the city and Baltimore Gas and Electric, which occupies 75% of the conduit, had generally blamed the fires on brittle infrastructure dating to the 19th century, which would require an extraordinary investment to entirely replace.

While the report noted the age of some portions of the conduit, the bigger concern was the overcrowded utilities below Baltimore’s streets.

Some of the consultant’s recommendations echo calls that members of the City Council have been making for nearly two years. At previous hearings on the topic, Councilwoman Odette Ramos had called for the city to share maps detailing underground infrastructure. If no map existed, Ramos suggested the city make one.

Without some sort of risk assessment program, more fires are possible.

City Councilman Mark Conway, who chairs the public safety committee and has held hearings on the underground fires, issued a statement suggesting BGE customers could be on the hook for some of RTI Group’s recommended fixes.

Scott reached a deal with BGE during his first term that made the utility responsible for capital improvements to the city’s conduit system.

“A major consequence of that structure is that necessary safety improvements are no longer treated as municipal infrastructure work,” said Conway, who is also running for Congress.

He said he would call further council hearings examining the issue.

This story has been updated.