Nogales, Mexico - The number of undocumented migrants detained near the United States' border with Mexico fell by almost a third in 2015, US government data showed.

Arrests by the Border Patrol fell by 30 percent to 337,117 in fiscal year 2015 - the second lowest number since 1972, according to data released by the Department of Homeland Security. It marked an 80 percent drop from the peak reached in 2000.

"Overall this decrease indicates that the investments that we've made in border security are proving effective and that we are seeing fewer attempts to illegally cross the border," said a senior DHS official on condition of anonymity.

Of the 337,117 people apprehended near the US border, 188,122 were Mexican, 57,160 Guatemalan, 43,564 Salvadoran and 33,848 Honduran. The number of minors intercepted dropped sharply, by 41 percent, to 39,838 in 2015.

Homeland Security Secretary Jeh Johnson called it a "dramatic decrease ... reflecting a lower level of attempted illegal migration at our borders."

There are nearly 12 million unauthorized immigrants in the United States, many of them Mexicans who have lived and worked in the country for many years. According to DHS data, the United States deported a total of 462,463 people from the country in 2015.

In the new fiscal year, Johnson said, "we will be challenged again by a variety of factors driving illegal migration to the U.S., mostly from Central America, and we are redoubling our border security efforts now to meet that challenge."

For the third straight year, migrant deaths along the border have decreased. U.S. Customs and Border Protection reported that there were 240 migrant deaths at the border for the fiscal year that ended in September, down from 308 deaths last year and 365 deaths five years earlier.