By Cynthia Lin 
 

Gains in U.S. Treasurys on Friday pushed the 10-year yield to its lowest level in four weeks, but wasn't enough to claw the market out of a monthly loss.

Benchmark 10-year notes gained 10/32 in price to yield 2.097%, the lowest closing level since April 30. Two-year notes gained 1/32 to yield 0.605%. Bond yields decline when prices rise.

Prices rose as a sharp drop in Chicago-area factory activity fueled demand for safer bonds, while typical month-end purchases from money managers squaring positions supported Treasurys.

"Factors in Treasurys in the month of May have been more technical in nature, but economic fundamentals are still somewhat positive for the market," said Gary Pollack, head of fixed-income trading at Deutsche Bank's private wealth-management arm.

Dan Mulhollad, senior U.S. Treasury trader at Credit Agricole, said Friday's rally was driven by a particularly large month-end effect. Money managers who benchmark their bond portfolios to indexes have to align their positions when new bonds are worked into those benchmarks at the close of each month.

That so-called index extension for May is estimated to be the largest since August 2009, bond traders and strategists said.

Still, May has been a rough month for Treasurys despite lackluster U.S. economic data that would normally support the safe-haven bonds. The market started the month in the middle of an 8-day selling streak as eurozone government bonds sold off on concerns of an overstretched rally.

A wave of bond offerings from U.S. corporations also took a toll on Treasury prices. The 10-year yield hit a 2015 high of 2.37% on May 12.

For the month, Treasurys have handed investors a total loss of 0.38% through May 28, according to Barclays. That reduced the market's total return so far this year to 0.7%.

Bond investors look for U.S. economics to regain control of the market in June, as long as eurozone bonds remain calm and ongoing negotiations between Greece and its creditors don't strike a nerve.

The Commerce Department early Friday said the U.S. economy shrank 0.7% in the first quarter instead of expanding 0.2% as originally reported. While investors are hopeful for a spring rebound, traders say it will take a string of positive U.S. data to further chip away at Treasurys and pull forward expectations for when the Federal Reserve will start tightening policy.

"While the sticker shock of a negative 1Q GDP print [result] could prove supportive to Treasurys," other more up-to-date data will help shape Fed policy expectations, said Gennadiy Goldberg, a U.S. strategist at TD Securities.

Fed Chairwoman Janet Yellen's latest comments suggest the central bank is still on track to raise interest rates this year--an event that is keeping bond investors on edge.

Many investors started the year thinking the Fed would tighten policy by midyear. But after the weak first-quarter performance, expectations have been pushed back to September and December, while some don't even see a move until early 2016. The Fed's policy-setting board is scheduled to deliver an updated policy statement and economic projections on June 17.

 
    COUPON   ISSUE    PRICE      CHANGE     YIELD    CHANGE 
    5/8%     2-year  100  1/32   up 1/32    0.605%   -2.0BP 
    1%       3-year  100  6/32   up 2/32    0.930%   -1.2BP 
    1 1/2%   5-year  100  1/32   up 3/32    1.490%   -1.9BP 
    1 3/4%   7-year  100  0/32   up 3/32    1.873%   -1.0BP 
    2 1/8%   10-year 100  8/32   up 10/32   2.097%   -3.5BP 
    2 1/2%   30-year 102  26/32  up 17/32   2.859%   -3.7BP 
    2-10-Yr Yield Spread: +160.1BPS +169.2BPS 
 
   Source: Tradeweb/WSJ Market Data Group 
 
 

Write to Cynthia Lin at cynthia.lin@wsj.com