A surge of undocumented children and families from Central America detained at the US border could trigger a repeat of the 2014 migrant crisis just as the presidential campaign gathers pace.

Border agents detained 21,469 people travelling in family groups in the last three months of 2015 – almost triple the number held during the same period in 2014, according to new figures released by Borders and Customs Protection. The vast majority were from the northern triangle of El Salvador, Guatemala and Honduras, where authorities are struggling to cope with drug-fuelled violence, corruption and institutional breakdown.

The number of unaccompanied children more than doubled to 17,370, compared with just under 7,987 in the last three months of 2014. The apprehension of 6,782 children in December made it the fifth highest month for child detentions on record. Undocumented crossings are usually low in December because of the holidays and cold weather.

The figures reveal the sharpest rise in vulnerable women and children seeking refuge at the US border since the summer of 2014. Then, images of women and children locked up in inadequate detention centres triggered a public outcry and led Barack Obama to declare the flow of undocumented Central American migrants into the US a humanitarian crisis.

A subsequent wave of patchwork measures – including beefed-up border security, advertising campaigns in Central America warning people against travelling to the US, and the multimillion-dollar Southern Border Programme (Plan Frontera Sur) to apprehend migrants in Mexico – were implemented in lieu of comprehensive immigration reforms that Republican lawmakers opposed.

The numbers reaching the US overland halved almost immediately as Mexican authorities doubled the number of migrants it detained and deported.

But the number of families and children arriving at the southern border has been rising steadily since August 2015.

“The steady monthly increase despite Mexico’s continued efforts, makes me think this pattern is probably permanent, and that the sophisticated smuggling networks operating with the help of corrupt officials have adjusted to the Southern Border Plan,” said Adam Isacson, a security analyst at the Washington Office on Latin America (Wola).

“Going into spring we’re going to see lots of children and families coming into American communities which anti-immigration candidates will try to take advantage of as we head into the primaries. Migration is already a polarising election issue … these numbers are going to reverberate in the campaigns.”

Immigration is always a key issue in American elections, but the popularity of Republican maverick Donald Trump and his anti-immigrant rhetoric has propelled it to centre stage. Trump and his closest rival Ted Cruz support building a wall along the US-Mexico border to keep undocumented migrants out.

The latest detention figures, while still lower than during the height of the 2014 crisis, show Obama’s package of measures did little to tackle the root causes driving the exodus, according to Mike Allison, associate professor of political sciences at Scranton University and author of the Central American Politics blog.

Many are fleeing violence. The murder rate in El Salvador increased 70% last year to make it the most dangerous peacetime country in the world. The tiny nation of 6.5 million recorded a murder rate of 104 per 100,000 habitants amid soaring gang bloodshed and extrajudicial killings.

The Peace Corps this week announced it was abandoning its work in El Salvador because of insecurity. In the past three months, 12,000 Salvadorans sought refuge at the US southern border.

“There have been no meaningful improvements in the security, political or economic situations in the Northern Triangle which would make people reconsider abandoning their homes,” Allison said.

An unprecedented $750m aid package approved by Congress last month to help restore law and order in the three countries by rebuilding weak institutions such as the police and judiciary is unlikely to yield positive results for at least a decade.

The need for a more immediate deterrent may have triggered recent immigration raids across the US in which 121 people were detained, and at least 80 summarily deported. Democrat frontrunners Hilary Clinton and Bernie Sanders have both criticised the crackdown.

According to Isacson, the raids were an obvious attempt to look tough and head off anti-immigration critics. “We need to ensure people have access to due legal process because if they’re fleeing violence, they’re not doing anything illegal.”