Rosa Isela Guzmán Ortiz, Guzman's eldest daughter, lives in California with her four children. Earlier this year, she told The Guardian that her father crossed the U.S. border from Mexico twice in 2015 to see a new house he bought.
During a Finance Committee hearing with Customs and Border Protection Commissioner Gil Kerlikowske, Grassley asked if Guzman's daughter was interviewed after she said her father entered the United States while he was a wanted fugitive.
"I don't know whether the daughter was interviewed by Customs and Border Protection," Kerlikowske told Grassley on Wednesday.
On Thursday, Grassley continued his inquiry by sending a letter to Homeland Security Secretary Jeh Johnson and FBI Director James Comey.
"I asked Commissioner Kerlikowske yet again if the federal government had any knowledge that 'El Chapo 'entered the United States ... he was unable to assure the Committee that it thoroughly vetted the allegation or possibility that 'El Chapo' entered the country undetected," Grassley wrote in the letter. "Further, Commissioner Kerlikowske admitted that he was unsure whether 'El Chapo's' daughter had even been interviewed by authorities during the course of the investigation."
Guzman's Sinaloa Cartel, according to reports, is the biggest player in the illegal drug market in the United States. "El Chapo" -- meaning "The Short One" or "shorty" -- so dubbed because of his 5-foot-6-inch frame, was detained in Guatemala in 1993 and then extradited to Mexico to face murder and drug trafficking charges.
"Accordingly, please respond as to whether anyone in your department has interviewed Rosa Isela Guzman Ortiz and provide any information relating to interviews or investigations conducted by Homeland Security Investigations or the Federal Bureau of Investigation regarding this incident," Grassley wrote.
Last week, a Mexican federal judge ruled drug lord Joaquin "El Chapo" Guzman be extradited to the United States, though the final decision rests on Mexico's Ministry of Foreign Affairs.
Guzman escaped from prison in 2001 by hiding in a laundry cart after bribing prison guards, and was re-captured in February 2014. He was captured in the city of Los Mochis in his home state of Sinaloa on Jan. 8 after escaping from Mexico's Altiplano Federal Prison on July 11, 2015.