— -- The firearms used in last week's shooting at a Virginia park where Republican congressmen were practicing baseball were purchased legally by the alleged gunman, according to multiple law enforcement sources.
That reportedly follows a pattern, as many of the guns used in recent mass shootings were also obtained lawfully.
According to a database maintained by Mother Jones, there have been at least 90 mass shootings in the United States since 1982, and most of the shooters got their guns legally. The database focuses on what the publication calls "indiscriminate rampages in public places resulting in four or more victims killed by the attacker," and excludes shootings stemming from more conventional crimes such as armed robbery or gang violence.
Of the 143 guns wielded by killers in mass shootings, more than three quarters were obtained legally, including dozens of assault weapons and semi-automatic handguns with high-capacity magazines, according to Mother Jones.
In January of 2013, Mother Jones also began including shootings in which three or more people died, after a federal mandate for investigations of mass shootings lowered the victim baseline from four to three.
Other databases differ in both their guidelines and tallies of mass shootings.
Gun Violence Archive, which collects data from shooting incidents, defines a mass shooting as an incident where four or more people are wounded or killed, not including the perpetrator. By that definition, it tallies at least 160 mass shootings already in the first five and one half months of 2017.
Here’s a timeline based on news reports of some public shootings over the past two years in which authorities confirmed the weapons were obtained legally:
Alexandria, Virginia: Five people injured on June 14, 2017
A 7.62 mm caliber SKS rifle and a 9 mm handgun were used in the terrifying shooting at a baseball field in Alexandria, Virginia, on the morning of June 14. Authorities recovered the guns from the alleged attacker and found shell casing from both weapons on scene, according to Andrew Vale, assistant director in charge of the FBI's Washington field office.
Vale said the suspected shooter, identified by authorities as 66-year-old James T. Hodgkinson, legally purchased the SKS rifle in March 2003 and the handgun in November 2016 through federal firearms licensees.
Multiple law enforcement sources told ABC News he was able to purchase both weapons legally because he had no felony convictions, though he reportedly had several previous run-ins with the law.
Hodgkinson, who also owned a shotgun, is believed to have obtained a permit in approximately the last two months to carry a concealed weapon, sources told ABC News.
At a news conference Wednesday morning, Vale said Hodgkinson "acted alone” and the FBI so far doesn’t believe there is any nexus to terrorism, though the motive remains unknown.
Five people were injured in the shooting, including House Majority Whip Steve Scalise, whose condition was upgraded from "serious" to "fair" on Wednesday afternoon. Hodgkinson died from injuries sustained while exchanging gunfire with authorities at Eugene Simpson Stadium Park, which is about seven miles from the White House.
Fort Lauderdale, Florida: Five people killed on Jan. 6, 2017
Esteban Santiago, a 26-year-old Iraq War veteran, is accused of going on a murderous rampage at an airport in Fort Lauderdale, Florida, that left five people dead and six others injured on Jan. 6. Santiago allegedly began his shooting spree in a baggage claim area with a gun that federal law enforcement officials told ABC News was checked legally in his luggage.
The combat engineer had received a discharge for unsatisfactory performance at the rank of E3 – or Private First Class – on Aug. 16, 2016.
Federal authorities told ABC News that Santiago had recently received mental health treatment after he showed up at the FBI’s office in Anchorage, Alaska, claiming that the government was forcing him to watch videos produced by the ISIS terror group. In that incident, Anchorage police found Santiago's gun in his vehicle, held it until he was released four days later and contacted him about picking up the weapon, which he did on Dec. 8, according to a Chicago Tribune interview with the Anchorage police chief.
Orlando, Florida: 49 people killed on June 12, 2016
Omar Mateen, 29, allegedly killed 49 people and wounded 53 others at Pulse, a gay nightclub, in Orlando, Florida, in the early morning of June 12, 2016. After a shootout with police and taking clubgoers hostage, Mateen was shot and killed by authorities.
Mateen purchased an AR-15-style rifle and a handgun from a federally licensed dealer near his home in Fort Pierce, Florida, on separate days about a week before the attack. He passed a full background check and had two security-officer licenses, one of which allowed him to be armed while on duty, according to Ed Henson, owner of the St. Lucie Shooting Center.
Mateen was investigated by the FBI in 2013 and 2014, after his co-workers expressed concerns about statements he had made concerning possible ties to terrorist groups. Neither inquiry led to criminal charges.
Kalamazoo, Michigan: Six people killed on Feb. 20, 2016
Jason Brian Dalton, a 45-year-old former Uber driver, was arrested after he allegedly opened fire in three locations in Kalamazoo, Michigan, killing six people and injuring two others on Feb. 20, 2016. Dalton faces trial on charges of murder, assault with intent to commit murder and using a firearm during the commission of a felony.
Dalton allegedly used two legally purchased 9 mm semi-automatic handguns in the killing of two people at a Kia car dealership and four people outside a Cracker Barrel restaurant. Police say he bought the firearms at a local gun shop in 2015 and had no disqualifying criminal or mental health history. It's unclear how Dalton obtained 14 other guns that authorities said federal agents later seized while searching his home.
San Bernardino, California: 14 people killed on Dec. 2, 2015
Police say Syed Farook and his wife, Tashfeen Malik, shot and killed 14 people and injured 22 others at the Inland Regional Center in San Bernardino, California, on Dec. 2, 2015. Four hours after the rampage, the couple was shot dead by police in a gun battle on a San Bernardino street.
Farook and Malik used assault rifles that the FBI says were lawfully purchased by Farook’s neighbor from a licensed dealer in 2011 and 2012.
The neighbor, Enrique Marquez, pleaded guilty in February to conspiring with Farook in 2011 and 2012 to provide material support and resources to terrorists, including weapons, explosives and personnel. He is scheduled to be sentenced in August.
Marquez told investigators that Farook asked him to buy the weapons because he would draw less attention. At the time, the FBI says, the men were plotting to shoot up a community college and a highway.
Roseburg, Oregon: Nine people killed on Oct. 1, 2015
Christopher Harper-Mercer, 26, allegedly opened fire at Umpqua Community College in Roseburg, Oregon, on Oct. 1, 2015, killing nine people and wounding nine others before killing himself. Harper-Mercer and his family members legally purchased the handguns and rifle he used in the murderous rampage from a federally licensed gun dealer, according to the federal Bureau of Alcohol, Tobacco, Firearms and Explosives.
Roanoke, Virginia: Two people killed on Aug. 26, 2015
Police say Vester Lee Flanagan II, 41, shot and killed two journalists during a live on-air interview in Roanoke, Virginia, on Aug. 26, 2015. The woman being interviewed was injured in the shooting. After being apprehended by police, Flanagan shot himself and later died at a local hospital.
Federal officials say Flanagan bought the handgun he used in the attack legally from a licensed dealer. He had not been convicted of a crime or determined to be mentally ill.
Lafayette, Louisiana: Two people killed on July 23, 2015
Police say John Russell Houser, 59, opened fire inside a packed movie theater in Lafayette, Louisiana, on July 23, 2015, killing two people and injuring nine others before taking his own life.
Investigators say Houser used a semiautomatic pistol he had legally purchased from a pawn shop in Alabama. He had been denied a state-issued concealed weapons permit in 2006 because he was accused of domestic violence and soliciting arson.
Chattanooga, Tennessee: Five people killed on July 16, 2015
Muhammad Youssef Abdulazeez allegedly killed five people and wounded two after he opened fire at a pair of military installations in Chattanooga, Tennessee, on July 16, 2015. Authorities said Abdulazeez was killed in a gunfight with police.
According to the FBI, some of the weapons Abdulazeez used in the attack were purchased lawfully and some were not. It is unclear when the purchases were made and whether he was subject to a background check.
Relatives told ABC News that Abdulazeez was deeply troubled and had a history of mental illness. They say he had been arrested prior to the shooting on suspicion of driving under the influence of alcohol. And in May 2013, he failed a background check for an engineering job at a nuclear power plant in Ohio.
Charleston, South Carolina: Nine people killed on June 17, 2015
The FBI says Dylann Roof, 21, should have never been allowed to purchase the pistol with which he killed nine people and injured one other at the historic Emanuel AME Church in Charleston, South Carolina. Roof was convicted of hate crimes resulting in death, among other charges. He was sentenced to death.
When Roof tried to buy a gun from a dealer in West Columbia, South Carolina, in April 2015, an FBI background check examiner found a felony drug charge in his record. But the examiner missed that Roof had actually admitted to possessing the drugs during his March 2015 arrest. That admission should have barred Roof from buying the gun, according to the FBI.
After a three-day wait, the gun dealer was legally permitted to complete the transaction. Two months later, Roof went to a Bible study group at the church and opened fire with the gun.
The Associated Press contributed to this report.