Military bases across the United States have been put on high alert in the wake of Friday's mass shooting at Navy Station Pensacola.

US Northern Command, also known as NORTHCOM, sent out an advisory calling for an increase in security checks on Saturday night, according to Fox News.

It comes as the FBI continues to hunt for several Saudi military students from Pensacola, who have seemingly vanished in the wake of Friday's attack.

Fellow Saudi Mohammed Saeed al-Shamrani, 21, killed three and wounded 12 others at the base before he was shot dead by police.

Investigators have now detained 10 Saudi military students from the base - a number revised up from six - but several others remain unaccounted for.

Authorities have not revealed the number of Saudi students they are still looking for, and they have not stated whether they are a risk to the public.

The New York Times reports that al-Shamrani and three fellow Saudi students traveled from Pensacola to New York last week, where they visited several museums and are thought to have watched the Rockefeller Center Christmas tree lighting ceremony.

The tree lighting, attended by thousands of people, took place on Wednesday evening - just 36 hours before the shootings occurred.

According to one source, investigators are probing the motive for the group's trip to the city. They also want to know who the men met with while they were there.

Meanwhile, sources with knowledge of the investigation say that three Saudi serviceman joined al-Shamrani for a dinner party on Thursday night to watch videos of mass shootings.

In the hours leading up to the attack, the shooter appeared to have posted criticism of U.S. wars in the Middle East to social media, saying he hated Americans for 'committing crimes not only against Muslims but also humanity' and for the country's support of Israel.

He also posted a quote from assassinated al Qaeda leader Osama bin Laden, according to SITE Intelligence Group. The shooter's Twitter account was taken down subsequent to the attack.

Mohammed Saeed al-Shamrani, 21, killed three and wounded 12 others in the attack before he was shot dead by police. This picture was released by the FBI on Saturday evening

It has also been reported that one Saudi student allegedly videotaped al-Shamrani's attack, while two others watched from a nearby car. It's believed that those three Saudis are among the 10 who have been detained.

While Friday's shooting his has not yet been officially deemed a terror attack, FBI terrorism investigators have been pictured investigating at the Pensacola base.

US Naval Academy graduate, Joshua Kaleb Watson, 23, (left) and military student Mohammed Haitham, 19, (right) have been identified as two of the victims of Friday's shooting

Naval apprentice Cameron Scott Walters, 21, was named as the third victim

The latest: Military bases are put on high alert

US Northern Command, also known as NORTHCOM, issued the alert on Saturday in the wake of Friday's attack at Pensacola, and a separate, unrelated attack at Pearl Harbor in Hawaii on Wednesday.

A sailor whose submarine was docked at Pearl Harbor, Hawaii, opened fire on three civilian employees Wednesday, killing two before taking his own life.

'Given the recent attacks at two military installations, the Commander, US Northern Command has directed all DoD [Defense Department] installations, facilities and units within the US Northern Command area of responsibility to immediately assess force protection measures and implement increased random security measures appropriate for their facilities,' Lieutenant Commander Michael Hatfield told Fox News.

'The advisory also told leaders to remind their workforce to remain alert and if they see something, to say something by immediately reporting to appropriate authorities any suspicious activity they may observe,' Hatfield said.

The FBI Evidence Response Team is pictured continuing their methodical search for clues at the base on Saturday. FBI terrorism investigators have been also been investigating, according to reports

Naval Air Station Pensacola will remain closed until further notice, officials said Saturday. The building where the shooting took place is pictured

Who was gunman Mohammed Saeed al-Shamrani?

Al-Shamrani was a second lieutenant attending the aviation school at Navy Station Pensacola. The Pentagon say his training with the US military began in August 2016, and was due to finish in August 2020.

AL- SHAMRANI'S DISTURBING TWITTER ACCOUNT AND HIS PRE-SHOOTING 'MANIFESTO' The now-deactivated Twitter account purportedly belonging to al- Shamrani included: - A variety of anti-Israel postings and a quote from deceased al-Qaeda leader Osama bin Laden - A lengthy manifesto posted at 4:39am Friday, less than two hours the shooting. The manifesto read in part: 'I'm against evil, and America as a whole has turned into a nation of evil. 'I'm not against you for just being American, I don't hate you because [of] your freedoms, I hate you because every day you [are] supporting, funding and committing crimes not only against Muslims, but also humanity.... Advertisement

One of al-Shamrani's uncles told CNN on Saturday that he was shocked by the attack, as his nephew was 'likable and mannered towards his family and the community'.

'He had his religion, his prayer, his honesty and commitments', the uncle stated.

Meanwhile, it's been revealed that al-Shamrani used a handgun in the shooting, which he purchased from a dealer in Pensacola.

Non-citizens are prohibited from purchasing guns in the United States, unless they are equipped with a hunting license.

According to NBC, al-Shamrani was equipped with such a license.

The gun has been described as a Glock 45 9-millimeter handgun with an extended magazine.

al-Sharami allegedly had four to six other magazines in his possession at the time of his shooting.

Elsewhere, the FBI is examining social media posts and investigating whether al-Shamrani acted alone or was connected to any broader group.

On Friday evening, the SITE Intelligence Group, which monitors jihadist media, claimed they had tracked a Twitter account belonging to al- Shamrani which featured a disturbing manifesto written just hours before the shooting.

Investigators are working to determine if it was in fact written by the shooter

Mohammed Saeed al-Shamrani was a Saudi aviator training at the U.S. naval station

SAUDI ARABIA'S TERROR LINK 15 of the 19 men associated with the al-Qaeda 9/11 attacks were Saudi citizens. A number of them received their aviation training at bases in the US. al-Qaeda's leader, Osama bin Laden, hailed from a prominent Saudi family. The US investigated some Saudi diplomats and others with Saudi government ties who knew hijackers after they arrived in the US, according to documents that have been declassified. The Saudi government has repeatedly denied any involvement in the attacks. The 9/11 Commission report found 'no evidence that the Saudi government as an institution or senior Saudi officials individually funded' the attacks that al-Qaeda masterminded, but the commission also noted 'the likelihood' that Saudi government-sponsored charities did. In 2017, families of 800 victims and 1,500 first responders file a lawsuit against Saudi Arabia accusing the country's officials of aiding hijackers in 9/11 attacks. The lawsuit is ongoing. Saudi Arabia is the richest and geographically largest Middle Eastern country - and a key US ally. According to the State Department, Saudi Arabia is the second leading source of imported oil for the United States, providing just under one million barrels per day of oil to the US market Saudi money is also widely invested in the US, with billions of private cash invested on Wall Street and beyond. Prince Alwaleed bin Talal, a member of the ruling al-Saud family, has stakes in a huge range of big businesses including Citigroup, 21st Century Fox, and the Plaza Hotel in New York. Many the country's citizens also come to study at US colleges, with Saudis the fourth-largest source of foreign students, trailing only China, India and South Korea, according to The New York Times. The US has long had a robust training program for Saudis, with 852 Saudi nationals currently in the country training under the Pentagon's security cooperation agreement. Saudi Arabia was not one of the seven countries included in President Trump's 2017 'travel ban' Advertisement

The three victims are publicly named

US Naval Academy graduate Joshua Kaleb Watson, 23, military student Mohammed Haitham, 19, and naval apprentice Cameron Scott Walters, 21, have been named as the three men who were shot and killed by al-Shamrani.

Watson was the first victim to be named, with his family confirming his death on Saturday morning.

In a heartbreaking tribute on Facebook, Watson's brother wrote that he 'saved countless lives today with his own.'

'After being shot multiple times he made it outside and told the first response team where the shooter was and those details were invaluable. He died a hero,' wrote brother Adam Watson.

Watson was a native of Enterprise, Alabama who was actively involved in JROTC and National Honor Society in high school.

On Saturday evening, the family of Haitham also confirmed that he was among the three killed.

Haitham, known as 'Mo' to those who knew him, was a track and field star from Lakewood, Florida, according to the Tampa Bay Times.

He graduated from high school in 2018 and joined the Navy soon afterward. Haitham completed boot camp and was assigned to flight crew training in Florida.

Evelyn Brady, his mother who herself is a Navy veteran and who now works for the Veterans' Administration, said she was informed of her son's death.

'The commander of his school did call me,' she said.

'He told me my son did try to stop the shooter.'

Meanwhile, naval apprentice Walters, 21, from Richmond Hill, Georgia was confirmed as the third victim late Saturday.

Friends paid tribute on social media, saying he was a 'kind-hearted' and 'wonderful' person.

A SURVIVOR'S STORY Among the 12 who were wounded was airman Ryan Blackwell, 27, who works at Naval Base Pensacola processing paperwork for international students. Ryan Blackwell, 27, was shot and injured on Friday He remains in intensive care after being shot in his pelvis and his right arm . His intestines were severed by a ricocheting bullet. Speaking to Pensacola News Journal from his hospital bed on Saturday, Blackwell recalled how he and his two colleagues were shot through a glass door. 'My adrenaline was pumping so much,' he recalled. 'I was worried about getting us to safety and getting us out of there' Blackwell and his colleagues jumped out of a window and ran to safety, despite the fact they had all been shot and were 'bleeding out'. 'We could have been three more casualties if we didn't escape,' Blackwell recalled. Advertisement

How US and Saudi officials have reacted to the attack

On Saturday, President Donald Trump told reporters: 'We're finding out what took place, whether it's one person or a number of people. We'll get to the bottom of it very quickly.'

He had faced criticism on Friday evening for his initial response to the shooting, which perceived as a defense of Saudi Arabia.

After news of the shooting broke, Trump tweeted his condolences to the families of the victims and noted that he had received a phone call from Saudi King Salman.

'The King said that the Saudi people are greatly angered by the barbaric actions of the shooter, and that this person in no way shape or form represents the feelings of the Saudi people who love the American people,' Trump wrote.

Secretary of State Mike Pompeo said on Saturday that he had spoken with Saudi Foreign Minister Al-Saud, who 'expressed his condolences and sadness' at the shooting.

However, not all US politicians have been as conciliatory.

Florida Governor Ron DeSantis suggested Saudi Arabia should offer compensation to the victims.

'The government of Saudi Arabia needs to make things better for these victims, and I think they're going to owe a debt here given that this is one of their individuals,' DeSantis said on Friday.

Florida Governor Ron DeSantis suggested Saudi Arabia should offer compensation to the victims

Naval Air Station Pensacola: Home of The Blue Angels

One of the Navy's most historic and storied bases, Naval Air Station Pensacola sprawls along the waterfront southwest of the city's downtown and dominates the economy of the surrounding area.

Part of the base resembles a college campus, with buildings where 60,000 members of the Navy, Marines, Air Force and Coast Guard train each year in multiple fields of aviation.

The base is also the home of the Navy's revered Blue Angels aerial demonstration team

Weapons are not allowed on the base Naval Air Station Pensacola, which will remain closed until further notice.