KANSAS CITY, Mo. (AP) -- A retired University of Kansas professor was alarmed when the federal government first opened his mail several years ago. Now that it's happened again, he wants answers.

"I want to know why," said Grant Goodman, 88. "I want to know why me?"

Goodman, a University of Kansas professor emeritus of history who specializes in Japan and southeast Asia, said U.S. Customs and Border Protection opened a letter he received recently from a friend in the Philippines. The letter, which was in a white air mail envelope, arrived at Goodman's home with official green tape marked 10-27-12 and a message saying it had been opened by Customs.

In 2005, a letter Goodman received from the same friend in the Philippines was also opened by Customs and resealed with the same type of tape.

"I think it's very unpleasant and very disturbing and I have no idea why I am a seeming target or what their rationale is for their selection of the letters to open," he said in a telephone interview Tuesday from his home in Lawrence, Kan. "The strange thing is that these letters have been coming regularly. They could open any one. ... If they wait two weeks, they can see another."

Goodman, who taught at Kansas for about 30 years, said he doesn't know why Homeland Security would be interested in his correspondence with his friend, who he wouldn't identify by name because he doesn't want to involve her but said she is also in her 80s, educated and Catholic. He'd prefer to correspond with her via email but says she doesn't have a computer and has been sending him letters twice a month for decades. Each letter usually includes newspaper clippings to "to keep me up to date about their political situation, military activities, U.S.-Philippines relations."

There's nothing in the letters that could be considered suspicious, he said.

"I'm a historian of Asia," Goodman said. "And I lived in the Philippines at different times in my life. ... They're not unusual or excessive or provocative in any way whatsoever."

Federal law has long protected first-class domestic U.S. mail against search and seizure, according to the U.S. Postal Service website. If the Postal Service has reason to believe such mail could be illegal, it needs a warrant to open it.

But those rules don't apply to international mail. U.S. Customs and Border Protection, a federal agency under the Department of Homeland Security, has "a broad and unique border search authority to ferret out what is inadmissible and illegal," said spokeswoman Cherise Miles.

Miles said CBP officers don't need either a warrant or probable cause to examine international mail or other cargo.

All mail that comes into the U.S. from abroad is sent to U.S. Customs by the Postal Service. Customs inspectors examine the mail and decide if duty is owed and whether that mail warrants closer inspection.

Such searches led to the discovery of about 131,118 pounds of narcotics in the 2011 fiscal year, she said.

"They're really, really good at finding needles in the haystack, so to speak," Miles said.

Miles said, while she was not familiar with Goodman's case, his letters may have been inspected because they included items other than just a letter -- the newspaper clippings -- and came from the Philippines, which is considered a high-risk origin.

Goodman said he's left wondering if the next letter will get similar treatment.

"I think it's important that no one in the general public seems to have any knowledge of this," he said. "If they're going to do this, we should know about it."