Retired corrections officers at the news conference said inmates who are used to receiving lax oversight from corrupt officers in one facility might be more inclined to resent strict treatment in another.

"They don't like the fact that the officers do their job, don't let them break the rules," said former Lt. Steve Berger, who worked in another Western Maryland correctional facility. "Officers do get targeted because they do their jobs."

Binetti said five detainees from the Baltimore jail were transferred to North Branch, including White, though he was there only a short time.

Based on intelligence assessments, Binetti said, the other four transferred detainees are not considered "problematic."

Union officials gave reporters copies of the letter in which the inmate threatened the two officers at North Branch. They did not identify the inmate or the charges for which he is incarcerated.

The inmate did not explain his problem with the officers. He wrote to the tier captain as an equal.

"I come to you with much respect as we both are men of power over our troops," he wrote. He said he is "not the main man" for his organization, which he did not name, but said he does run security.

He also wrote that his "troops" would continue to target both officers — indicating that the officer who was not attacked could still be in danger.

But "I don't think you would be that stupid to put one of them in the line of fire again to get punished," the inmate wrote. "That wouldn't be a good chess move."

The prison remains volatile, Grabenstein said, and incidents such as the stabbing have affected corrections officers.

"It's dropped morale to an all-time low," he said, "which in itself creates more danger." He said officers who are "not on top of their game at all times" make themselves vulnerable.

Moran and Grabenstein said the department has taken some measures to address the violence.

Since the end of June, Binetti said, the North Branch facility "has either been on lockdown status, employed sectional or tier lockdowns, or utilized modified inmate movement schedules and activities, as necessary."

The prison has an estimated average daily prisoner population of about 1,400.

The department is reducing the size of recreation groups among prisoners, Binetti said.

But union officials said more must be done to limit the ability of maximum-security inmates to assault other inmates and guards.

Moran said Corrections Secretary Gary D. Maynard and other department leaders have to do a better job listening to "front-line staff" about ways to improve security, but didn't provide specifics.

Binetti said Maynard was at North Branch on Tuesday. He declined to make him available for an interview.

Binetti rejected assertions by Moran that the department has been using different standards for recording assaults in recent years to make the totals seem lower.

Binetti said the department has been using the same national standards since 2003.

The department's data show serious assaults on officers have trended down from 20 in fiscal year 2007 to 8 in fiscal year 2013. No officer has been killed since 2006.