(NewsNation) — Department of Homeland Security Secretary Alejandro Mayorkas testified before the Senate as Republicans continue a push to hold the Biden administration accountable for what they call a crisis at the border.

In his opening statement Mayorkas first expressed support for those affected by recent tornadoes and for the family and friends of those killed in a school shooting in Nashville, Tennessee. He painted a picture of a “heightened security environment,” citing global conflicts and tension as well as concerns of violence and domestic terrorism.

The influx of fentanyl coming into the U.S. was a top concern for senators on both sides of the aisle. Mayorkas brought up the use of technology and efforts by the department to use artificial intelligence as a force multiplier for border patrol agents. In a tense exchange with Sen. John Cornyn, Mayorkas repeatedly emphasized ports of entry as a major concern.

“Ninety percent of fentanyl is brought through ports of entry. Through vehicles, through trucks, through pedestrians,” Mayorkas said.

Sen. Mike Lee accused Mayorkas as redefining status by not detaining migrants seeking asylum or deporting them if detention centers are at capacity. The secretary also pushed back on the idea that his agency had the unilateral authority to institute a safe third country arrangement, where migrants who passed through a third country on their way to the U.S. would be deported if they had not first sought asylum in that country.

Mayorkas declined to answer a question on whether or not his department had evidence of China conducting surveillance through social media app TikTok, citing legal proceedings currently underway. He also faced heavy questioning regarding an increase in encounters with Chinese nationals who entered the country and the possibility that those people could have ties to the Chinese Communist Party.

The Senate Judiciary Committee hearings come after earlier testimony from Border Patrol Chief Raul Ortiz, who said he believed the U.S. does not have control of the border. Border security has been a top GOP issue, as they criticize the Biden administration for handling migrants who attempt to cross illegally.

But numbers of migrant encounters have actually begun trending down. Data released by U.S. Customs and Border Protection paints a picture of who is attempting to enter the country at a time of intense political controversy, with Republicans seeing immigration as a potent issue with voters and accusing President Joe Biden and Mayorkas of not doing enough to secure the southern border.

U.S. Border Patrol officials encountered migrants trying to cross the border 128,877 times in February between the legal border crossings. That’s about the same as January’s number — 128,913 — and is the lowest number of encounters per month since February 2021, the agency said.

The numbers of encounters doesn’t necessarily equate to individual people since some migrants try repeatedly to cross the border. The agency said about 25% of those encountered in February were repeat encounters, meaning that at some time during the last 12 months they’d been detained by U.S. officials as they tried to enter the country.

In comparison, U.S. officials stopped migrants 221,693 times between the ports of entry along the Mexican border in December.

The Associated Press contributed to this report.