Washington (CNN) President Donald Trump is deploying the National Guard to patrol the border and hopes to get Congress to fund thousands more Border Patrol agents. But there are already thousands of those jobs unfilled.

According to Customs and Border Protection, as of March 17, there were 19,346 Border Patrol agents on duty, short of the congressionally mandated 21,370.

That's almost 100 fewer agents than there were at the end of September 2017.

The agency's website is full of postings for career fairs as it tries to fill its ranks. The 2018 government spending bill said that CBP expects to lose more agents than it will hire this year, and accounted for that in its funding.

Congressional appropriators also noted that the agency's request for more Border Patrol agents "was not supported by any analysis of workload and capability gaps" that would allow them to assess whether the agents were even necessary. Border crossings were at historic lows last year , though they have rebounded somewhat.

In fact, Customs and Border Protection has been struggling for years to even keep up with the rate of agents quitting or retiring, let alone hire the thousands more Trump has called for.

In a memo asking for more hiring authorities obtained by CNN last year, now-Commissioner Kevin McAleenan laid out the difficulties of hiring, concluding that optimistic estimates show it would still take five to 10 years to hire the 5,000 additional agents Trump has sought.

In that memo, McAleenan said attrition was 6% a year, meaning it would take 2,729 border patrol agents being hired annually to reach just full staffing in five years.

The administration has made clear it does not intend to relax standards for the agency to hire more personnel, but institutional factors have made it difficult to attract enough candidates.

A 2010 anti-corruption bill passed by Congress required polygraph tests for all agents, concerned about the possibility they could be blackmailed, bribed or compromised by gangs, cartels or criminals. Lawmakers have proposed legislation that would relax the system only somewhat, waiving the polygraph for certain veterans and former security clearance holders.

The agency also struggles to compete with federal law enforcement agencies for talent. Many of the assignments are in small towns or isolated regions along the US border, and job demands can be strenuous and dangerous.