WASHINGTON - A CBS News investigation has found sexual misconduct in the ranks of U.S. Customs and Border Protection (CBP) is "significantly" higher than other federal law enforcement agencies, according to the agency's former head of internal affairs.

In April, one agent in Arizona was indicted for sexually assaulting a 5-year-old child off duty. The allegation is just the latest in a string of cases where agents -- on or off duty -- have been accused of sex crimes against women or children.

For eight years James Tomsheck handled sexual misconduct investigations as the Chief of Internal Affairs at U.S. Customs and Border Protection. What he saw, he says, disturbed him.

James Tomsheck, the former Chief of Internal Affairs at U.S. Customs and Border Protection CBS News

Between 2012 and 2014, he found there was a "spike" of more than 35 sexual misconduct cases against agents, a rate he says was significantly higher than other law enforcement agencies.

"A problem that I believed was a significant and serious problem was perhaps even worse than I thought it was," said Tomsheck.

Last April Tomsheck emailed his boss, the head of the agency, Commissioner Gil Kerlikowske, about the "disturbing" and "disproportionate" number of cases "in the agency's workforce."

Tomsheck says he didn't receive a response.

A CBS News investigation found in the last six years, at least 21 customs or border patrol agents have been indicted or pleaded guilty to sexual offenses -- both on and off duty -- ranging from sexual assault, to possession of child pornography and child molestation. Five agents were arrested in the past year.

CBS News

Tomsheck says some of the people involved in the cases he found had the potential of being serial offenders.

"I believe they were persons who had likely offended before but their acts have gone undetected," said Tomsheck.

Tomsheck, a 23-year veteran of the Secret Service, blames the problems in part on a lack of screening for more than 15,000 agents hired between 2006 and 2012 who were never given polygraphs -- now standard procedure for new hires at the agency.

According to Tomsheck, one of those not polygraphed was agent Esteban Manzanares. Police believe Manzanares picked up a woman and two teenage girls trying to sneak into the U.S. at the border along the Rio Grande, sexually assaulted them and attempted to kill two of them. He later brought one of them back to an apartment and raped her. When the FBI knocked on his door, Manzanares committed suicide.

Tomsheck believes all agents should take a polygraph every five years - a requirement in the FBI.

"I believe there are many persons in the organization today that if subjected to periodic polygraph screening would be found to be unsuitable," said Tomsheck.

Tomsheck says after he brought this problem to light he was pressured to resign and retired early. CBP declined an on-camera interview but said the agency has received 285 documented complaints or allegations since 2009 related to sexual misconduct; 44 are still open ongoing investigations. The agency says last month it adopted a zero tolerance policy on sexual abuse.

This story was produced by Laura Strickler and Luisa Garcia