WASHINGTON — The Department of Homeland Security agreed to stop confiscating prescription drugs mailed to American consumers from Canadian pharmacies, Senator Bill Nelson said Tuesday. Mr. Nelson, a Florida Democrat, had asked the Senate Committee on Homeland Security and Government Affairs in June to investigate the seizing of prescription drugs by Customs and Border Protection agents. The drugs had been bought by Americans. The decision to stop the confiscations, which became effective Monday, means that the Food and Drug Administration resumes the job of overseeing drug imports from Canada. Mr. Nelson still wants a Congressional investigation ‘to seek answers on why the administration started the medicine seizures in the first place,’ his health counsel, Jon Cooper, said Tuesday in an interview. The senator raised the issue after being contacted by Lee and Jean Edes of Mount Dora, Fla., who discovered that drugs they were ordering from Canada were vanishing in the mail, having been seized by federal agents.