WASHINGTON — (CNN) Officers Montrell Jackson, Matthew Gerald and Brad Garafola were killed Sunday after being ambushed and shot by a lone gunman in Baton Rouge, Louisiana.

Their deaths have kept the spotlight on a region where the July 5 shooting death of Alton Sterling at the hands of police began what has been two weeks of national turmoil.

“We as a nation have to be loud and clear that nothing justifies violence against law enforcement,” President Barack Obama said after Sunday’s attack.

“Attacks on police are an attack on all of us, and the rule of law that makes society possible.”

Now police across the country are taking precautions.

The New York Police Department is doubling up all foot patrols and security posts, according to a memo obtained by CNN.

Officers are also being instructed to take all meals and personal breaks in pairs.

The Pittsburgh Bureau of Police is closely monitoring all calls for service and making sure backup is available under all circumstances.

In Cleveland, the police union asked Gov. John Kasich to restrict the state’s open carry laws temporarily for the Republican National Convention in light of the events in Baton Rouge.

For better protection, the Boston and Sacramento, California police departments are requiring that two officers be in every patrol unit.

The Indianapolis police are ordering officers to go through training on how to recognize an ambush before it happens and what to do if they find themselves the target of one.

Two officers in Baltimore Police Department must now respond to every call.

Several departments across the country told CNN that they are taking more safety measures but declined to describe them.

Although Baton Rouge is “hundreds of miles away from Boston, the pain and suffering caused by the loss of these officers in the line of duty is felt deeply by the men and women of the BPD,” Boston Police Commissioner William Evans said Sunday.

“This all too common trend we are seeing of violence against law enforcement officers who are out there each day serving and protecting neighborhoods across the country is alarming and disheartening.”

Obama urged a stop to the bloodshed.

“Only we can prove, through words and through deeds, that we will not be divided,” the President said Sunday. “And we’re going to have to keep on doing it again and again and again. That’s how this country gets united.”

The shooting Sunday took place around 8:40 a.m. (9:40 a.m. ET) in the city of about 230,000 people, already tense after the high-profile police shooting of Sterling, an African-American man.

On Sunday, police received a call of a “suspicious person walking down Airline Highway with an assault rifle,” a source with knowledge of the investigation told CNN.

Killer Gavin Long “ambushed” the officers, said Col. Michael D. Edmonson, superintendent of the Louisiana State Police.

Long’s “prey was those police officers,” Edmonson said Monday morning on CNN. “He drew them to the scene.”

After the first set of officers were attacked, more responded to the scene and killed Long, authorities said.

He had been carrying an AR-15-style, semi-automatic rifle, law enforcement sources told CNN.

At a Sunday afternoon news conference, local and state authorities, including Louisiana Gov. John Bel Edwards, said Long was thought to be the lone gunman.

Earlier reports had said authorities believed there might have been more than one attacker. Edwards described the shooting as an “absolutely unspeakable heinous attack.”

The Louisiana State Police said they questioned and released two people in relation to the attack, and no charges were filed.

Authorities are interviewing people that Long was speaking to while he was in Baton Rouge, Edmonson said. “We want to know what brought him here, what kept him here” and why he killed police, the superintendent said.

Edwards, also appearing Monday on CNN, reflected on the pain that officers’ relatives and co-workers endured at the hospital until late Sunday. “Emotions are raw,” the governor said. “There’s a lot of hurting people.”

Officers Jackson, 32, and Gerald, 41, both worked for the Baton Rouge Police Department.

Gerald had been serving for less than a year and, like Jackson, was assigned to the uniform patrol bureau, according to the department.

Jackson had posted on Facebook on July 8 how physically and emotionally drained he had been since protests had erupted in Baton Rouge after the killing of Sterling by police.

“I swear to God I love this city, but I wonder if this city loves me. In uniform I get nasty, hateful looks and out of uniform some consider me a threat. … These are trying times. Please don’t let hate infect your heart.”

Garafola, 45, worked for the East Baton Rouge Sheriff’s Office for 24 years, according to Casey Rayborn Hicks, a spokesman for the sheriff’s office.

The gunman also critically wounded a deputy who is “fighting for his life,” said East Baton Rouge Parish Sheriff Sid Gautreaux.

Another wounded deputy and police officer have wounds not considered life-threatening, law officers said.

Authorities later identified 41-year-old Nicholas Tullier as an officer in critical condition.

Bruce Simmons, a 51-year-old with 23 years of service, sustained non-life threatening injuries, authorities said.

Long, a black man from Kansas City, Missouri, was a Marine who was discharged as a sergeant in 2010.

CNN has filed a Freedom of Information Act request with the military to obtain records about Long’s service.

Under Defense Department rules, health records that might include any information on Long’s mental health are considered protected even though he is dead.

The records are part of the criminal investigation.

CNN has filed a similar request for the records of Micah Johnson, also formerly a military member.

He killed five police officers in Dallas, and a criminal investigation is ongoing about his military record.

Long served from August 22, 2005, to August 1, 2010, according to the little information the military has released.

He was a data network specialist who received the Marine Corps Good Conduct Medal, Navy Unit Commendation Medal and others.

He signed up for the service in Kansas City and deployed to Iraq from June 2008 to January 2009.

The releasable record on Long doesn’t indicate where he worked in Iraq.

He left a long trail of information online about his beliefs under the pseudonym Cosmo Setepenra.

Since the shooting death of Sterling by Baton Rouge police, the department has worried about threats against officers.

It has been an emotionally charged few days across the country because of protests stemming from Sterling’s death and the shooting by police of Philando Castile in Minnesota, plus the ambush on police officers in Dallas.

“This is an unspeakable and unjustified attack on all of us at a time when we need unity and healing,” Edwards said Sunday in the hours after the Baton Rouge shooting.

Quinyetta McMillon, mother of Sterling’s son Cameron, put out a statement Sunday through her attorneys condemning the attack.

“We are disgusted by the despicable act of violence today that resulted in the shooting deaths of members of the Baton Rouge law enforcement,” she said. “My hope is that one day soon we can come together and find solutions to the very important issues facing our nation rather than continuing to hurt one another.”