(J. Scott Applewhite - AP) At least five Senate and House district offices have received threatening mail in the last three days with a suspicious powdery substance — and similar letters with harmful materials may exist, congressional law enforcement officials warned.

The most recent letter was discovered Wednesday in a Senate state office, according to Senate Sergeant at Arms Terrance W. Gainer. The letters have tested negative for harmful substances and there have been no injuries, he said.

Gainer and House Sergeant at Arms Paul D. Irving informed all congressional offices via e-mail Wednesday to treat incoming mail with care and to be on the look-out for letters with “a suspicious powdery substance.”

Rep. Maxine Waters (D-Calif) (Melina Mara - THE WASHINGTON POST)

Updated 3:18 p.m.

A half-dozen members of the House Ethics Committee have recused themselves from the troubled investigation of Rep. Maxine Waters (D-Calif.) and been replaced by a new team, a sign that the stalled probe is set to restart.

Waters has faced scrutiny since 2009 over whether she improperly sought federal aid for OneUnited, a bank in which her husband had a financial stake. The Ethics panel has been riven by infighting, as Republican and Democratic members and aides accused each other of mishandling the investigation and Waters, who has maintained her innocence, has demanded the case be dismissed.

Last year, the committee hired veteran defense lawyer Billy Martin Jr. to conduct an outside review of the probe. In a letter Friday to Speaker John Boehner (R-Ohio), six committee members said Martin had found no evidence of any “actual bias or partiality” by the lawmakers, but they were voluntarily recusing themselves from the case anyway to “eliminate the possibility of questions being raised” and “move this matter forward.”

Waters’s office had not provided comment on the recusal as of this posting.

Chinese Vice President Xi Jinping on Wednesday fielded tough questions from U.S. lawmakers on a range of issues including China’s record on human rights, its valuation of its currency and its vote together with Russia against this month’s United Nations resolution on the conflict in Syria.

U.S. Senate Majority Leader Sen. Harry Reid (2nd R), Senate Minority Leader Sen. Mitch McConnell (2nd L), and Chinese Vice President Xi Jinping (C) pose for photographers prior to a meeting Feb. 15, 2012 on Capitol Hill. (Alex Wong - Getty Images)

The questions came in back-to-back meetings with top senators and House members on Capitol Hill Wednesday morning on the second day of Xi’s visit to Washington. Later Wednesday, he addresses an economic group in D.C. before heading to Iowa.

In an interview after Xi’s half hour meeting with top senators in the Senate’s Lyndon B. Johnson room, Sen. John McCain (R-Ariz.) said that most of the conversation had been focused on trade and intellectual property issues, and that he had been the sole lawmaker to raise the issue of China’s human rights record — including China’s U.N. vote on Syria.

“His response to my statements was, he said, ‘Senator McCain, your candor is very well-known in China,’” McCain said.

“I just mentioned to the vice president there’s been enormous and dynamic economic progress, but we still have Tibetan monks burning themselves to death; we have Nobel Prize winners under house arrest; and the continued propping up of North Korea, a brutal regime that is capable of causing profound international crises; and I mentioned to the vice president I thought it was really wrong to veto the resolution on Syria in the Security Council,” McCain said. “The Syrian government is massacring thousands of its people.”

As Chinese Vice President Xi Jinping visits Washington Tuesday, members of Congress and others are ramping up their calls for China to address a range of issues including the valuation of its currency, its human rights record and Iran’s nuclear program.

Chinese Vice President Xi Jinping, right, gives an opening statement near U.S. Vice President Joe Biden during talks at a hotel in Beijing, Aug. 19, 2011. (Ng Han Guan — Associated Press)The China currency issue is one that remains a hot topic on Capitol Hill, particularly as the United States continues its sluggish economic recovery. The Senate passed with overwhelming bipartisan support late last year a measure that would have given the Treasury Department greater latitude in pressuring China to allow the value of its currency, the yuan, to rise.

But the measure has stalled in the House, where Republican leaders have declined to bring the measure to the floor, arguing that the White House must first make its position known on the bill.

(Ben de la Cruz - THE WASHINGTON POST) Is it okay that congressional earmarks are sometimes used to fund projects near lawmakers properties?

In 2011, 33 members of Congress sent more than $300 million toward such projects, according to a recent Washington Post investigation.

On Wednesday, Post reporters David Fallis, Scott Higham and Kimberly Kindy held a Congress and earmarks live chat with readers to answer questions about Congress, earmarks and what this information means for your district.

[Post Investigation: Congress and Cash | Full Results]

Readers wanted to know if these earmarks were illegal — answer: no — and what makes this practice bad. This brought up an important distinction.

“We did not label an earmark as ‘good’ or ‘bad,’” Kindy said. “The stories point out flaws in the financial disclosure system. It should be very easy for voters to determine whether an earmark is by a lawmakers’ property or if an earmark went to an organization where a relative works. It is not.”

But there was still concern from at least one reader that the investigation was misleading, painting earmarking practice with a broad brush.