Eating lunch in uniform at his favorite spot on Friday, Chelsea police Officer Sammy Mojica sat facing the door.

You never know, he said, when someone might come in shooting.

“In today’s world, you just don’t know how to act anymore as a police officer,’’ said Mojica, who has been on the force for 17 years. “Sometimes we feel like our hands are tied behind our backs and people are out to get us.’’

Still reeling from news of an ambush that killed five police officers and injured six others in Dallas, some Boston-area officers acknowledged an unavoidable feeling — one that has long informed both sides of the relationship between police and black communities all over the country: Fear.

And some worry that this mutual fear — longstanding in the black community but stoked in recent days by the latest in a string of high-profile police shootings of black men — threatens to escalate already tense interactions.

“How can you not be afraid?’’ said Boston police Sergeant Mark Parolin, president of the Boston Police Superior Officers Federation. Parolin said he was worried the slayings in Dallas would lead to increased violence against police officers across the country.

“We’ve seen it here,’’ he said, pointing to the shooting last year of Officer John Moynihan on Humboldt Avenue in Roxbury. Moynihan was shot in the face but survived.

“Everybody’s watching each other’s backs,’’ he said. “That’s all we can do, is look out for each other.’’

Around the Boston area, police officers have expressed dismay over the shootings in Dallas. Cambridge police made social workers and the police chaplain available to on-duty officers.

Quincy police Captain John Dougan said Friday was a “hard, sad day for police officers to go to work.’’ Dallas was “an out and out ambush on police officers. They were out there protecting people,’’ he said.

“When a police officer is fallen, or wounded, that really hits home to us,’’ Dougan said.

Some worried that Thursday night’s events could heighten levels of distrust that were already high following the police shootings of Alton Sterling in Louisiana and Philando Castile in Minnesota.

“This is an incredibly challenging time in law enforcement. Relationships with communities of color have been frayed for a long time,’’ said Shea Cronin, an assistant professor of criminal justice at Boston University. “Law enforcement has always operated under the assumption that they work in risky positions, and they are always in danger. These kinds of events exacerbate it.’’

Police chiefs and other leaders in law enforcement should work to build connections with police reform movements, Cronin said.

“We know from research that when officers hold very distrustful views of the community, when they’re more cynical . . . they tend to have more complaints against them,’’ Cronin said. When police approach situations fearfully, they tend to be more aggressive and less patient; confrontations that could be contained escalate instead.

And in the same way that police shootings in other parts of the country are felt deeply here — evidenced by the growth of the Black Lives Matter movement following the 2014 death of Michael Brown in Ferguson, Mo. — an attack on officers elsewhere can foment fear and anger among police in departments down the street.

“When you see an event like this, it’s going to affect law enforcement everywhere,’’ Cronin said.

“Men and women who wear the uniform are feeling tremendous pressure from all sides,’’ said Arlington Police Chief Fred Ryan, who was scheduled to appear this weekend on a Harvard Law School panel about policing post-Ferguson. “There is no time in history where it’s more difficult to be a police officer.’’

Ryan said the best antidote to today’s challenges for police is transparency, building a culture of trust instead of fear.

“We’ve done a lot of training around unconscious bias — we have a robust program on prohibiting bias-based policing,’’ Ryan said. “We have to accept and embrace these challenges and overcome them.’’

Mojica, the Chelsea police officer, said he is buoyed somewhat by what he’s seen in youth programs, such as one he participated in on Friday morning.

“Before, they wouldn’t say ‘hi’ or acknowledge me,’’ Mojica said of the teens he’s met over the years. “Now, you should see them — they go out of their way to say, ‘What’s up.’ ’’

But he said something has changed in recent years.

“New officers must be a nervous wreck,’’ Mojica said. “When I first started, the job was fun. It’s not fun no more.’’

Evan Allen and Kathy McCabe of the Globe staff contributed to this report. Nestor Ramos can be reached at nestor.ramos@ globe.com. Follow him on Twitter @NestorARamos.