The Waltham apartment complex that burned in a massive fire Sunday morning did not have the type of proactive plan that experts say can help prevent fires at construction sites, because the city did not require such plans when the project was permitted in 2015.

But Waltham officials had recently conducted a safety inspection at the site, following a similar blaze last month in Dorchester, and found that nearly everything was being done according to code.

The cause of the 10-alarm blaze, which engulfed the 246-unit apartment complex under construction along the Charles River, is still under investigation, the state fire marshal’s office said.

Investigators on Wednesday asked for witnesses — including anyone with video or photos of the fire’s early stages — to come forward, though officials had said earlier that the fire did not appear suspicious.

Regardless, the Waltham fire, coming less than a month after an 83-unit building burned in the Ashmont section of Dorchester, highlights the concerns about an increasingly popular style of housing in Greater Boston: putting four or five stories of lightweight wood framing above a concrete ground floor. Fire officials warn that such tall wooden structures are especially vulnerable to fire during construction.

That risk could be lessened, some say, with proactive fire-safety plans. Such plans were recently written into state fire codes and are beginning to be required from builders by cities and towns.

The plans, called “NFPA 241’s,’’ a reference to a chapter in the National Fire Prevention Association’s book of standards, call for contractors to detail fire-safety protocols for construction projects — everything from rules about cleaning up debris and packaging to requirements that trained personnel be on watch during welding and other so-called “hot work.’’

Most require that someone be designated as a fire-safety manager. And the plans can call for more aggressive requirements, including around-the-clock security, temporary fire alarms, and even sprinklers — although most plans don’t.

“They’re customized to the project, and they put a lot of responsibility on the owner to ensure there’s proper planning in place for fire safety,’’ said Felix Zemel, a former state chief building inspector who now is a building code consultant. “The presence of a 241 would show that someone had really thought about fire safety during construction.’’

But while 241 plans have long been a part of building codes, they’ve only recently been required by cities and towns in Massachusetts, after a 2015 state law added them to the fire codes.

“That has put a heightened sense of awareness — and some teeth — into this,’’ said Peter Harrod, a fire-protection consultant with Code Red Consultants in Southborough. “And fire services can start to more clearly hold contractors accountable.’’

Since then, Boston and Cambridge have begun requiring 241 plans on large construction projects. Other municipalities are starting to do the same.

In 2015, when Waltham officials issued permits for Lincoln Property Co. and its partners to build 20 Cooper St. — the complex that was destroyed Sunday — the city did not mandate a 241 plan, Deputy Fire Chief Cliff Richardson said.

Callahan Construction, the general contractor, confirmed that it did not file a 241 plan, but a spokeswoman said Callahan “enacted its rigorous safety plan on this project — as is best practice on all projects managed by the company,’’ reflecting state codes for worksite safety. She could not provide details of the plan, but said the company’s site superintendent oversaw fire safety measures.

Lisa Nickerson, speaking for the project team, said: “Site security is a key priority and the firm is continually investing in leading means to best protect its job sites . . . Callahan consistently reviews its site security policies and procedures to ensure they meet and exceed industry standards, and is currently examining where even greater measures can be taken.’’

After the June 28 Ashmont fire, Waltham officials decided to take a closer look at several big projects under construction in their city, including 20 Cooper St. Inspectors swept through the site to ensure standpipes and fire extinguishers were up to code and that exit and no-smoking signs were properly installed.

“We found it was substantially in compliance with all state building code regulations,’’ said building inspector William Forte. “We were satisfied.’’

But Waltham is considering stronger regulations. Richardson said large projects now require a detailed 241 plan, and Forte said he’s starting talks with state and local fire and building officials “to come up with some sensible code changes that can prevent this in the future.’’

A 241 plan, however, does not guarantee fire safety at a building site. Cranshaw Construction had filed an eight-page 241 with the City of Boston to cover the apartment building that burned in Ashmont, but fire officials said this week that the inferno was sparked by a hot vent pipe installed to close to combustible materials.

Still, Harrod said, pushing builders to be more proactive about preventing fires is important. “A construction site is a dangerous place with lots of potential areas for accidents to happen,’’ he said. “We’re just trying to mitigate that risk.’’

Tim Logan can be reached at tim.logan@globe.com. Follow him on Twitter at @bytimlogan.