A series of suspicious packages on the Senate side of the Capitol put staffers, police and lawmakers further on edge and on alert about security concerns at the complex.

The combination of this week’s bombing in Boston followed by intercepted letters that tested positive for ricin poison addressed to Sen. Roger Wicker, R-Miss., and the White House gave way to chaos Wednesday, when suspicious packages were found in the Hart and Russell Senate Office buildings.

Capitol Police officers evacuated the premises, closed the entrances and put the whole area on lockdown. A few hours later the Capitol Police gave the “all clear,” but the Capitol campus remained on high alert and anxiety crescendoed to a general sense of panic.

Hours later, when Sen. Richard C. Shelby, R-Ala., confirmed to a group of reporters that his office had been the recipient of one of the suspicious letters and that initial testing had shown it to be “benign,” he encapsulated the theme of the day in just six words.

“In today’s world, you never know,” he said.

This week, security screening for non- credentialed visitors took longer. Outdoor garbage cans were turned on their sides and emptied, lest they be used as vessels for pipe bombs or other explosives.

It’s been nearly a decade since the last time Capitol Hill felt the heart palpitations of a terror scare. In the intervening years — several times a week, in fact — Capitol Police officers have been asked to respond to strange-looking parcels, unattended items and suspicious characters on the grounds. Sometimes areas are cordoned off or evacuated while checks are made.

Newer staffers and even members of the press who seized on Wednesday’s chaos in the Hart and Russell buildings wouldn’t know just how routine these incidents are because they often result in nothing. An overwhelming majority of the time, a suspicious package is simply a backpack a tourist accidentally left behind. Last week, officers investigated reports of a man with a gun outside the House side of the Capitol; it turned out to be an umbrella sticking out of a bag.

But after the attacks in Boston and the possibility that ricin-laced letters were sent to Wicker and Obama, lawmakers and staffers on Capitol Hill have felt the renewed anxiety of living in a world left vulnerable ever since the events of 9/11.

Senate Sergeant-at-Arms Terrance Gainer said Senate mail would not be delivered for the rest of the week, and he told staffers not to accept any hand-delivered mail unless it was from a uniformed, credentialed official.

The heightened concern wasn’t just at the Capitol, either. Two senators said their state offices had also received strange envelopes.

In a statement, Democratic Sen. Carl Levin said the letter that arrived at his Saginaw, Mich., office was being tested for hazardous material and that the staffer who had handled the piece of mail, while experiencing no symptoms, planned to stay overnight in a local hospital for observation.

Sens. Jay Rockefeller, D-W.Va., and Lindsey Graham, R-S.C., both said that the situation with mail delivered to state offices, which isn’t subject to the same kind of off-site scrutiny as mail sent to the Capitol complex itself, would be reviewed by aides in light of recent events.

“I think we’ve had a good measure of response here,” Graham said. His chief of staff will serve as the point person on the review of mail procedures.

Rockefeller said he did not yet know what his staff planned, but expected to hear back from his state office by the end of Wednesday regarding the procedures.

“My guess is yes,” there will be new procedures, “because of the person who runs the state office … they would do that, but I don’t know for sure.”

Some Senate staffers said informally that they wished they had received better communication about the developments of the past 48 hours. Email bulletins from Gainer’s office go only to a limited number of more senior Senate aides, and many staffers said they were left to gauge the severity of the situation by word-of-mouth, media reports and social media such as Twitter.

“For the majority of issues, I respect the preference of the Member’s leadership staff, which is to give the info to Chiefs of Staff, Staff Directors, Chief Clerks, and Administrative Managers,” Gainer said in an email to CQ Roll Call. “Those individuals have email access through their BlackBerry or other communication device. They know to whom to give the information and the urgency (or not) of the message. I trust their judgment.”

Excitement seemed confined to the Senate on Wednesday, as the House Sergeant-at-Arms’ office reiterated to CQ Roll Call late in the afternoon that it had not received any threats or suspicious mailings.

But many senators heading to the chamber to take decisive votes on amendments to legislation to overhaul the nation’s gun laws said they felt confident with the procedures in place to keep Capitol Hill safe.

Asked if his state offices would be making any adjustments to how they receive and handle mail, or if they would be exercising additional caution, Sen. Richard M. Burr, R-N.C., replied: “No more than we normally do.”