Police also say the 15-year-old Gladden should never have been able to get his hands on the weapon. He found it unsecured in his father's home, according to court documents.
The shooting has shed light on the gap between the regulation of handguns — often used in crimes — and "long guns" such as the double-barrel Western Field shotgun that police seized, a firearm more common on a hunting range or farm. It has also raised concerns about securing firearms around children, which is required under state law.
As the community searches for ways to prevent similar acts of violence, advocates and opponents of stricter gun control agree that there should be a focus on keeping guns out of the hands of juveniles. But even if a push emerged to increase regulation of long guns in Maryland, it's not clear what it might call for or whether it would be effective, said Del. Curt Anderson.
"I don't know what specific new law you'd want to pass," said Anderson, a Baltimore Democrat who sponsored a 2011 bill prohibiting felons from owning long guns. "We just need to make sure the laws we have are enforced and the people who violate them spend time off the streets."
Shotguns, rifles and other so-called long guns are not considered regulated firearms under Maryland law and do not have to be registered, said Greg Shipley, a spokesman for the Maryland State Police.
Regulated firearms trigger policies such as a weeklong waiting period before purchase. That only applies to handguns and a list of about four dozen types of assault weapons, including some varieties of powerful rifles and shotguns.
Gladden is accused of shooting 17-year-old Daniel Borowy, a special education student, on the first day of school. A school guidance counselor tackled and disarmed Gladden, officials said.
Gladden found the shotgun in the Middle River home of his father, Robert Wayne Gladden Sr., and took it because it was the only one he could get, according to Baltimore County police. He was "well aware" that his stepfather, Andrew Eric Piper, had a number of guns at his Kingsville home, but he could not get to them because they were locked up, police said.
Gladden dismantled the shotgun into parts small enough to hide from view as he rode a school bus Monday morning, police said. He later concealed the reassembled firearm under his clothes before opening fire in Perry Hall High's cafeteria, according to court documents.
Daniel Webster, co-director of the Center for Gun Policy and Research at the Johns Hopkins University, called the distinction between long guns and handguns in laws "illogical." Long guns have been considered separately largely because they are steeped in hunting and farming culture, rather than in street violence, he said.
"In my opinion, we've had gun policies relevant to long guns that had been driven more by politics than by concerns about public safety," Webster said.
John H. Josselyn, legislative vice president for the Associated Gun Clubs of Baltimore, said that the focus should be on why some people resort to violence to communicate unhappiness. Patrick Shomo, president of gun rights group Maryland Shall Issue, said he agreed.
"I don't know if there's anything that could have prevented [the school shooting] other than good parental supervision," he said. "Continued laws that look to regulate inanimate objects are not going to regulate behaviors."
Many long guns are only regulated by federal law, Shipley said. Purchasing them requires a relatively quick review under the National Instant Criminal Background Check System, but state laws on background checks and waiting periods don't apply.
While carrying a concealed handgun requires a permit — the subject of a controversial court ruling earlier this year — it is legal to carry a loaded shotgun in many public places, according to a letter Attorney General Doug Gansler's office wrote to a state lawmaker last year. They are illegal on school grounds.
It wasn't until a state law was passed in 2011 that it became a crime for a convicted felon to own shotguns or rifles — not just handguns or assault weapons. Piper, Gladden's stepfather, was arrested Monday and charged with keeping guns despite a previous conviction for grand theft.
Other state laws do apply to long guns — including those restricting minors' access.
Crime statistics help explain the regulatory focus on handguns. Of 272 shooting homicides in Maryland last year, four involved shotguns and two involved rifles, according to state crime data. In 2010, long guns were used in 14 homicides.
But long guns are seized by police at a rate disproportionate with their prevalence in crime, in part because they are conspicuous by nature. About 40 percent of the 7,700 firearms that police seized in Maryland last year were rifles or shotguns, according to the Bureau of Alcohol, Tobacco, Firearms and Explosives.
While handguns are overwhelmingly the most common weapon for criminals, long guns have been used in many high-profile shootings nationally and in Maryland over the years. That includes the Colorado massacres at Columbine High School in 1999 and in a crowded Aurora movie theater in July.
Shotguns were among the firearms used in mass shootings at the Holocaust Memorial Museum in Washington, D.C., in 2009 and at an Amish schoolhouse in Nickel Mines, Pa., in 2006. They were also used in a shooting just off the campus of Frostburg State University that killed one student in 2010, and in the death of an Elkton woman as part of what her husband called a planned murder-suicide in 2006.
"There has not been a great effort to say, 'We have too many people walking around the streets carrying shotguns and other forms of firearms,'" said Sen. Brian Frosh, a Montgomery County Democrat who chairs a committee that oversees gun policies. "I think it's a very serious public health problem."
Frosh said he sees the laws shifting toward bolstering gun rights, rather than establishing new restrictions. That includes a March federal court ruling that struck down a requirement that Marylanders have "a good and substantial reason" to carry a concealed handgun, he said. The state attorney general's office has asked a higher court to block the ruling.
"Do I see a movement to restrict the right of people to carry shotguns in public? Nope," Frosh said.
Gun rights advocates decried the shooting but said they hoped it wouldn't lead to a heavier burden on law-abiding firearm owners.
Some on the message boards of Maryland Shooters, a popular website for gun enthusiasts, used the incident as a reminder of the importance of keeping guns locked and away from children 15 and under, as required by state law. Baltimore County Police Chief James Johnson highlighted that restriction in a message to the public on Friday.
No charges have been filed against Gladden's father, Robert Wayne Gladden Sr.
But there was also debate among gun owners over whether the focus on security missed a deeper issue.
"So if this kid had walked in with a [knife] and stabbed someone would we secure our kitchen knives?," one person wrote on the message board Tuesday. "A lot of people grow up around 'un secured' firearms and do not go out killing people."
Gun regulations in Maryland
Handguns and assault weapons:
Require a seven-day background check before purchase
Must be registered with state police
Require a permit when carried in public
Shotguns, rifles and other long guns:
Require an instant background check through federal system before purchase
Do not have to be registered
Can be carried in public without a permit