Among the agents being deployed to sanctuary cities are members of the elite tactical unit known as BORTAC, which acts essentially as the SWAT team of the Border Patrol. With additional gear such as stun grenades and enhanced Special Forces-type training, including sniper certification, the officers typically conduct high-risk operations targeting individuals who are known to be violent, many of them with extensive criminal records.

The unit’s work often takes place in the most rugged and swelteringly hot areas of the border. It can involve breaking into stash houses maintained by smuggling operations that are known to be filled with drugs and weapons.

In sanctuary cities, the BORTAC agents will be asked to support interior officers in run-of-the-mill immigration arrests, the officials said. Their presence could spark new fear in immigrant communities that have been on high alert under the stepped-up deportation and detention policies adopted after Mr. Trump took office.