WASHINGTON – The motive behind the mass shooting in Las Vegas Sunday night has yet to be determined by authorities, though Democrats already are jumping on the gun-control bandwagon.

Over the last 20 years, the perpetrators of nearly all the deadliest mass shooting in the United States have shared one of two traits: Besides killing innocents with firearms, they either were Muslims or were using mind-altering psychiatric drugs.

Stephen Craig Paddock is alleged to have opened fire from a 32nd-floor hotel room on a crowd of more than 22,000 gathered for a country music festival Sunday night, killing at least 59 people and injuring at least 515 others, making it the deadliest mass shooting in modern U.S. history.

The death toll surpassed the 49 killed last year in Orlando by Omar Mateen at the Pulse nightclub.

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Paddock killed himself in his hotel room before officers found his body alongside at least 16 firearms, law enforcement officials said.

Paddock's brother, Eric Paddock, revealed Monday that their father, Patrick Benjamin Paddock, was a bank robber who escaped prison and was on the FBI's Most Wanted list. The elder Paddock was "diagnosed as psychopathic" and "reportedly had suicidal tendencies."

Eric Paddock told CBS that Stephen had no history of mental illness, but he had a severe gambling addiction.

Meanwhile, ISIS is claiming Paddock was one of its followers, although no evidence has been found.

While Paddock's motive is still a mystery, the following is a look at some of the deadliest mass shooters in the U.S. over the last two decades, highlighting their common factors:

1. James Hodgkinson, congressional baseball practice (2017):

Hodgkinson, a Bernie Sanders supporter who reportedly "hated Republicans," opened fired at a GOP congressional baseball practice on June 14, wounding Rep. Steve Scalise, R-La., two Capitol Police officers and one congressional staffer.

His brother, Michael, told the New York Times that James was upset about the election of Donald Trump and moved to the Washington, D.C., area "out of the blue" to protest.

A list of Republican names in Hodgkinson's pocket was recovered by the FBI after he was fatally wounded in an exchange of fire with police.

Timothy Slater, special agent in charge of the FBI's Washington Field Office, revealed that Hodgkinson was taking prescription drugs, but he did not disclose what the drugs were for or whether he was abusing them.

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2. Esteban Santiago-Ruiz, Fort Lauderdale airport (2017):

Santiago-Ruiz shot five people to death and injured six others in Fort Lauderdale's Hollywood International Airport Jan. 7, 2017, near the baggage claim.

He served in the Alaska National Guard. He received a general discharge for unsatisfactory performance in August 2016.

The Iraq War veteran claimed his mind was being controlled by a U.S. intelligence agency and that he was being forced to watch videos for ISIS, according to the FBI. He was sent to a psychiatric hospital.

Relatives said he had been receiving psychological treatment.

The Fort Lauderdale shooter was also reportedly speaking to online jihadists and practicing shooting his pistol for months before he opened fire.

3. Omar Mateen, Orlando, Florida, Pulse nightclub (2016): 49 killed

The heavily armed gunman killed 49 people inside a gay nightclub in the city of Orlando on June 12, 2016.

Mateen was killed in a gun battle with police. He had pledged allegiance to ISIS, which later claimed responsibility for the attack.

It is being widely reported in the media that Mateen had bipolar disorder. His ex-wife, Sitora YuSufi, says Mateen claimed he was bipolar.

A resident of PGA Village in Port St. Lucie, Florida, where Mateen worked as a security guard, claimed Mateen was worn out days before the shooting because he repeatedly stayed up all night doing research on psychiatric medication.

"He wasn't as friendly. He was obsessed with researching medication online," the acquaintance told Reuters.

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4. Rizwan Farook and Tashfeen Malik, San Bernardino (2015): 14 killed

A newlywed couple, U.S. citizen Rizwan Farook and his Pakistani wife Tashfeen Malik, who was a permanent resident, left their 6-month-old baby with a grandmother while they stormed a holiday party at the Inland Regional Center in San Bernardino, California, killing 14 people and injuring 22 others.

They were shot and killed by police.

Farook was a devout Sunni Muslim.

Former FBI Director James Comey concluded that Farook and Malik were radicalized and likely inspired by foreign terrorist organizations.

5. Adam Lanza, Sandy Hook (2012): 26 killed

A 20-year-old American citizen, Lanza killed his mother in December 2012 before shooting and killing 20 children and six adults at Sandy Hook Elementary School in Newtown, Connecticut. He later committed suicide.

According to the website AbleChild, Paul Fox, the psychiatrist who treated Lanza was arrested for sexual misconduct and charged with three felony counts of sexual assault on a then-19-year-old patient for distributing the victim a "dynamic cocktail of psychiatric drugs" and for questionable billing practices and patient records retention.

It is unclear what psychiatric diagnoses or what kind of psychiatric "dynamic cocktail" Lanza was prescribed while a patient under Fox's "care" because the state of Connecticut refuses to release Lanza’s mental health records or autopsy and toxicology results.

Fox told detectives who were investigating the Sandy Hook massacre in 2012 that he had little memory of Lanza and destroyed any records he had of his treatment of Lanza prior to 2012.

6. Seung-Hui Cho Virginia Tech (2007): 32 killed

Cho, a 23-year-old student and South Korean national, went on a rampage at Virginia Tech University in April 2007, killing 27 students and five teachers before committing suicide.

The New York Times has reported the killer was on a prescription medication, and authorities said he was confined briefly several years before the attack for a mental episode.

7. Nidal Hasan, Fort Hood military base (2009): 13 killed

After jumping up on a desk and shouting "Allahu akbar," U.S. Army psychiatrist Nidal Hasan opened fire and sprayed more than 100 bullets inside a crowded building where troops were preparing to deploy to Afghanistan in November 2009. He killed 13 people and injuring 42 others.

Hasan reportedly was disciplined prior to the shootings for pushing his beliefs on others. His business card carried an abbreviation for "Soldier of Allah."

U.S. intelligence had been aware of email communications between Hasan and the Yemen-based terror organizer Anwar al-Awlaki. He wrote of al-Awlaki as his "mentor" and spoke out against the wars in Afghanistan and Iraq.

8. James Holmes, Aurora, Colorado (2012): 12 killed

Holmes, a U.S. citizen born in California and a graduate student in neuroscience, stormed a movie theater airing a late-night premiere of a "Batman" film in Aurora, Colorado, in July 2012. He was wearing body armor when he opened fire and released tear gas.

Twelve people were killed and 70 others wounded. Holmes was sentenced to life in prison.

Holmes was using two mind-altering psychiatric drugs during the night of the shooting, including the antidepressant Sertaline, generically known as Zoloft.

Zoloft has an FDA "black box" warning, the strongest warning the agency issues, cautioning that the drug can increase the risk of suicidal thoughts and behavior. Other dangerous side effects of Zoloft include agitation, irritability, anxiety, hostility, aggressiveness, hallucinations, mania, impulsivity, delusions and apathy.

Holmes was also on Klonopin, a drug that was initially designed to treat epileptic seizures by lowering electoral activity in the brain. It shares the same side effects of Zoloft but also can reportedly cause unpredictable reactions in people.

Forty-eight containers of beer and other alcohol were found in Holmes' apartment, prompting suspicion that he was likely drinking and taking his medications during the same time period, which would drastically increase the side effects of psychotropic drugs.

9. Jiverly Antares Wong, New York immigrant center: 13 killed

A Vietnamese immigrant, Jiverly Antares Wong, shot and killed 13 people at an upstate New York immigration center in April 2009 before killing himself.

Police concluded that Wong was distraught over losing a job and frustrated about his poor English language skills.

Wong displayed had no history of mental illness, but a letter he sent to a Syracuse television station revealed he was harboring a growing paranoia. Just before his killing spree, Wong sent a two-page delusional rant to the TV station saying the police were spying on him, sneaking into his home and trying to get into car accidents with him.

10. Aaron Alexis, Navy Yard headquarters (2013): 12 killed

Alexis, a 34-year-old information technology employee at a defense-related computer company, used a valid pass in September 2013 to get into the Navy Yard and then opened fire for more than a half hour, killing 12 people before he was slain by police in the shootout.

U.S. law enforcement found no evidence to suggest Alexis' motive was political or religious.

Alexis never sought care from a mental health specialist. He was never declared mentally ill by a judge or committed to a hospital, according to the Veteran Affairs medical centers.

But less than a month before Alexis went on the shooting rampage, he visited U.S. Department of Veteran Affairs medical centers twice, seeking treatment for insomnia. He was given medication to help him sleep, but when asked by doctors, he denied having thoughts about harming himself or others.

In August 2013, the former member of the Navy Reserve complained to Rhode Island police that people were talking to him through the walls and ceiling of his hotel rooms and sending microwave vibrations into his body to deprive him of sleep.

11. Eric Harris and Dylan Klebold Columbine High (1999): 12 killed

Harris and Klebold, two American teenage boys, shot and killed 12 classmates and a teacher, wounding 26 others before killing themselves at Columbine High School in Littleton, Colorado, in April 1999.

Harris was reportedly rejected by Marine Corps recruiters days before the Columbine High School massacre because he was under a doctor's care and had been prescribed antidepressant medications Luvox, Prozac, Paxil, Zoloft and Effexor, which he was on at the time he opened fire at Columbine High School.

Klebold's medical records remain sealed, but at least one public report exists of a Klebold friend witnessing him take the antidepressants Paxil and Zoloft.

12. George Hennard, Texas restaurant (1991): 22 killed

In October 1991, 35-year-old Hennard, a U.S citizen, shot dead 22 people in a restaurant in the town of Killeen before shooting himself.

Hennard of Belton, Texas, was angry that women kept rejecting him. He drove his car through the window of a restaurant and began firing, killing 14 women and eight men.

Hennard had suffered from a paranoid personality disorder, and experts said he was at "the very border of mental illness."

He said "treacherous female vipers" were trying "to destroy me and my family."