The ease with which Stephen A. Marshall boarded a bus that took him from Bangor to Boston on Easter Sunday while armed with two handguns and wanted for murdering two registered sex offenders is raising concerns about security on the nation’s buses.

Even if screening of intercity bus passengers will never approach the system in place to protect air travelers, Marshall’s case reveals a need to upgrade bus security, said Jeff Monroe, Portland’s transportation director.

“The biggest single weakness we have right now is the Department of Homeland Security is not focused on this,” said Monroe, who is working with transportation companies in Portland to draft a letter to Homeland Security Secretary Michael Chertoff seeking more funding for bus security.

Marshall, 20, of North Sydney, Nova Scotia, shot himself in the head after police stopped the Vermont Transit bus and closed in on him at Boston’s South Station. Investigators said Marshall had researched Maine’s online sex offender registry and killed two men on the list at their homes in Milo and Corinth that morning.

Buses are a vital component of the nation’s transportation system. They carry 774 million passengers a year, more than airlines and passenger trains combined, according to the American Bus Association.

Anti-terrorism planners regard buses as “soft” targets, meaning they are vulnerable because of relatively little security.

Security measures advocated by the bus industry include random passenger and baggage screening, controlled access at major terminals, security training for drivers, communications networks that link buses and bus systems, and background checks for workers.

Transportation Security Administration funding has led to some improvements, including an anti-hijacking barrier with a see-through door that separates the driver from the passengers.

“It would give the driver time to pull the bus off” if anyone tried to take control, said Greyhound driver Robert Spear.

Like Greyhound, Vermont Transit has installed a radio communications system for its buses, along with shields to protect drivers. But such measures cost money, said Chris Andreasson, Vermont Transit Lines’ general manager.

“The amount of money made available to the bus industry is minuscule compared to what airlines get,” he said.

The American Bus Association is seeking $50 million a year for three years, an increase from the recent level of $10 million a year.

Minjung Oh, 18, who was aboard the bus with Marshall when he killed himself, said the incident left her wary of buses and strangers.

Oh, a former Portland resident who lives in Boston, said she plans to buy a car this summer. “I’ll pay the money to be all by myself.”