After explosions and fires rocked the Merrimack Valley in September, state lawmakers now want to know if natural gas lines in Massachusetts are safe.

At a joint House and Senate hearing on the safety of the state's gas infrastructure Tuesday, lawmakers questioned executives and top engineers from each of the state's gas utility companies, but deliberately avoided investigating the specific Columbia Gas incident in Andover, North Andover and Lawrence that left one dead and thousands without service.

Hopkinton Rep. Carolyn Dykema challenged executives from National Grid, Eversource and other utilities for lacking standards when it comes to gas safety.

"What are we talking about when we say the system is safe? And it sounds like there's a tremendous amount of variability into how your operations are run," Dykema said.

Even though it wasn't explicitly on the agenda, the Columbia incident, in which federal investigators laid a degree of blame on Columbia subcontractors in an initial report, was on the mind of legislators looking to safeguard the state from leaks and explosions.

Read more: Transportation Safety Board Says Columbia Gas Could Have Prevented Merrimack Valley Fires

During hours of testimony, Sen. Michael Barrett grew frustrated with Columbia President Steve Bryant insisting the company is dedicated to safe gas line work.

"That's not a fact that's a contention, right? I mean, it's not fact. We can't demonstrate it with a metric," Barrett told Bryant after the executive mentioned Columbia's "culture" of safety.

Though the hearing was about the statewide gas network and not September's disaster in the Merrimack Valley, Bryant faced harsh criticism for not having enough safety workers on the job.

"We're committed to do what is necessary to create the safest environment possible. If that involves increasing staff, we will do that," Bryant told the panel.

After the hearing, Barrett said the state isn't in a position to safely oversee the gas industry and the Legislature may step in with new regulations as developers continue to connect new buildings to aging gas lines.

The House and Senate have queued up a bill to increase oversight of gas companies, suggesting that the days of utilities policing themselves may be over.