WASHINGTON — Federal agents arrested a man on Monday suspected of sending letters feared contaminated by the poison ricin to President Obama and a Mississippi senator, according to two officials with knowledge of the case. The suspect was identified as Paul Kevin Curtis of Tupelo, Miss.

The arrest, two days after the letters were intercepted in mail-sorting facilities for the White House and the Capitol, was based on information collected “very early on” about who had sent the letters, said one of the officials. The letters contained a postmark from Memphis but no return address, Senate officials said.

Tupelo is the hometown of Senator Roger Wicker, the Republican senator to whom one of the letters was addressed. The letters were signed: “I am KC, and I approve this message.”

The speedy arrests in the case may calm nerves in the nation’s twitchy capital, where it had begun to feel like the fraught weeks after the Sept. 11, 2001, terrorist attacks, when anthrax-laced letters were mailed to media organizations and two Democratic senators, killing five people and making 17 others sick.