Chief Michael Fisher Courtesy of Border Patrol

The national chief of the U.S. Border Patrol says allegations made last week by Cochise County Sheriff Larry Dever that Border Patrol agents are being instructed not to arrest all illegal border crossers to keep apprehension numbers down are 100-percent false.

In a letter dated Monday, Border Patrol Chief Michael Fisher wrote that he read Dever's comments to Fox News with "enormous disappointment and concern."

"The assertion is completely, 100 percent false and, most disturbingly, it unfairly casts a negative light on the hard work done each day in service to the nation and at risk to themselves by the men and women of the Border Patrol," Fisher wrote. "That it comes from a fellow law enforcement official makes it especially offensive."

"Law enforcement and border security decisions made at the operational level require the apprehension and arrest of every illegal border crosser," Fisher wrote. "Your unwarranted allegation to the contrary is just wrong."

You can read the full letter in the box to the left.

Here is an excerpt of what Dever said last week:

“The senior supervisor agent is telling me about how their mission is now to scare people back,” Dever said in an interview with “He said, ‘I had to go back to my guys and tell them not to catch anybody, that their job is to chase people away. … They were not to catch anyone, arrest anyone. Their job was to set up posture, to intimidate people, to get them to go back.”

Last week, Local 2544, the Tucson Sector chapter of the agents' union, the National Border Patrol Council, backed Dever's allegations in a post on their website:

"Sheriff Dever is right. We have seen so many slick shenanigans pulled in regards to "got-aways" and entry numbers that at times it seems David Copperfield is running the Border Patrol. Creating the illusion that all is well and you can start having family picnics in the areas where we work has been going on far too long. Has there been improvement in some areas? Absolutely. Is the border anywhere near "under control"? Absolutely not."

Dever's allegations and Fisher's letter mark the latest chapter in the ongoing back-and-forth between the Democratic Obama administration and its Democratic Secretary of Homeland Security, Janet Napolitano, and Arizona Republican Sheriffs Larry Dever and Paul Babeu, of Pinal County.

Napolitano has repeatedly said during speeches that the border is more secure than ever, citing an "unprecedented" increase in agents, fences and technology and resulting dip in apprehensions. She also cites FBI crime data that shows U.S. border cities are safer than ever.

But Dever and Babeu don't agree the border is more secure, with Dever calling the figures cited by Napolitano as “meaningless statistics that reflect nothing in terms of quality of life.” Dever points to the still-unsolved March 2010 murder of Cochise County rancher Robert Krentz as evidence the border is not more secure.

In February, Dever stepped down from a post representing Arizona sheriffs in a collaborative border security initiative with federal officials after Customs and Border Protection Commissioner Alan Bersin said during a press conference in Tucson that the border is safer and more secure, crediting a 1 1/2 -year-old initiative for recent progress in Arizona.